Several cities across the United States have begun to break new ground in recent years by legally recognizing polyamorous relationships. Somerville, Massachusetts, led the way in 2020, becoming the first city to pass a domestic partnership ordinance that includes relationships among more than two adults, granting them many of the same rights and privileges as married couples. Cambridge, Massachusetts, and Berkeley, California soon followed with their own measures, and Somerville has since introduced an antidiscrimination ordinance that explicitly protects people in polyamorous and other non-traditional families. These local efforts, combined with rising academic and advocacy interest in the legal status of multi-partner families, have sparked a pressing constitutional question: since Obergefell v. Hodges recognized same-sex marriage under the Fourteenth Amendment, does the Constitution also require recognition of plural marriage?

No Constitutional Right to Plural Marriage

Current substantive due process and equal protection doctrines do not recognize a constitutional right to plural civil marriage. Under the history-and-tradition framework established in Washington v. Glucksberg and Dobbs v. Jackson Women’s Health Organization, plural marriage is not a fundamental right.

Monogamous marriage is not simply a historical artifact or disguised moralism. It serves legitimate state interests. These include reciprocal and exclusive support obligations, shared responsibility for dependents, and a clear allocation of rights and duties between spouses and the state.

First, plural-marriage claims fail under the history-and-tradition methodology set forth in Glucksberg and Dobbs. There is no fundamental right to plural civil marriage. The Court’s modern substantive due process cases, read in light of the long-standing rejection of polygamy and plural marriage in American constitutional history, foreclose recognition of such a right.

Second, equal protection analysis shows why laws limiting civil marriage to two persons are subject to rational-basis review rather than heightened scrutiny, even if they incidentally burden people in consensually non-monogamous or polyamorous relationships. Under this deferential standard, plural-marriage restrictions satisfy the Equal Protection Clause because of the state’s interests in the administrability of family law, protection against certain intra-household vulnerabilities, and maintenance of a stable, predictable civil-marriage regime.

Third, a “mutual belonging” framework, based on Aristotle’s Nicomachean Ethics, provides an account of the nature of monogamous, dyadic marriage and the state’s interests in maintaining this institution. Introducing the concept of “mutual belonging” requires a precise explanation of what civil law protects in recognizing a two-person marriage and demands that this boundary be justified as principled rather than arbitrary, especially given the emergence of polyamorous-partnership ordinances.

Substantive Due Process: No Fundamental Right to Plural Marriage

Glucksberg and Dobbs provided the steps for analyzing substantive due process. The claim must meet a high standard: the right must be “deeply rooted in this Nation’s history and tradition” and “implicit in the concept of ordered liberty.”

Plural marriage has consistently been denied legal recognition. Marriage in the United States has always been limited to two people. Bigamy is void and criminalized, and polygamy falls outside the legitimate scope of civil marriage. In the nineteenth century, as civil marriage norms were established, Congress enacted strict anti-polygamy laws in the territories, and the Supreme Court in Reynolds v. United States upheld criminal prohibitions on polygamy, even against Free Exercise challenges.

A “mutual belonging” framework defines marriage as an exclusive, reciprocal union: a two-person commitment where each spouse both gives and receives, creating a legal ‘we’ rather than separate ‘I’ relationships. Marriage law has long favored this structure, establishing a two-person status based on reciprocal and exclusive support duties, shared responsibility for dependents, and a clear allocation of rights, authority, and obligations between spouses and the state. In Glucksberg/Dobbs terms, “mutual belonging” reflects the historical understanding of “ordered liberty” in marriage as a dyadic union, rather than as an unrestricted right to any consensual relationship.

Unity and exclusivity are the core organizing principles of marriage. Unity is foundational, while indissolubility is secondary. This unity is comprehensive and dyadic: a union of two people in which each spouse gives and receives exclusively from the other. What has historically been described as a “one flesh” relationship cannot be shared without altering its essential nature.

This framing is important for substantive due process because it aligns with the level of generality required by Glucksberg and Dobbs. The historically recognized right is not marriage in any form, but a specific civil status that is exclusive, reciprocal, comprehensive, and, critically, two-person.​

As physical attraction fades, “mutual belonging” matures: spouses remain connected as two people who share memories, burdens, and decisions over time. This is not unique to one marital “style” but is the standard focus of family law—an exclusive dyadic partnership that structures responsibility and reliance. The continuity of this dyadic form explains why the “expansion” arguments in Loving v. Virginia and Obergefell do not apply to plural marriage. Loving struck down interracial-marriage bans, and Obergefell recognized same-sex marriage, but both preserved the institution’s core structure. Plural marriage would fundamentally alter the institution by replacing dyadic unity with simultaneous multi-spouse status.

Advocates for plural marriage cite Obergefell to argue that if the Constitution protects marriage as a fundamental liberty interest grounded in individual autonomy, dignity, and intimate association, then the state cannot deny civil-marriage recognition to multi-partner unions merely because they depart from the traditional two-person form.

However, Obergefell did not constitutionalize plural marriage. It expanded access to an existing institution without changing its structure. The decision consistently describes marriage as a “two-person union” and frames the constitutional issue as exclusion from that dyadic institution based only on the sex of the partners. Plural civil marriage is therefore a distinct asserted right. Same-sex marriage maintains dyadic exclusivity, while plural marriage changes the institution’s numerical and structural core by allowing multiple simultaneous spouses.

Equal Protection: Plural-Marriage Bans Survive Rational Basis Review

Equal protection analysis begins by identifying what the law classifies as a class. Plural-marriage bans do not target identity categories. Instead, they regulate the simultaneous entry into more than one marriage. The rule is numerical and structural: no more than two civil spouses at any one time, applicable to all persons. Any adult, regardless of sexual orientation, religion, or relationship preference, may marry one person, but no adult may have multiple concurrent spouses under state law.

This framing is important because it precludes the application of heightened scrutiny. Heightened scrutiny applies to laws targeting suspect or quasi-suspect classifications such as race, national origin, sex, or, in some circuits, sexual orientation. Plural-marriage bans do not fit these categories. “Polyamorous” is not recognized as a suspect or quasi-suspect class, and the rule does not deny access to an institution for a specific group. It defines the institution’s permissible structure.

Therefore, rational-basis review applies. The ban is upheld if any legitimate governmental interest is rationally served by the two-person limit. Some argue that plural-marriage bans are similar to laws invalidated for hostility toward gay people. However, the “animus” cases are structurally different.

In Romer v. Evans, the Court invalidated a sweeping state constitutional amendment that uniquely denied gay people from seeking ordinary antidiscrimination protections, which the Court characterized as an impermissible “bare . . . desire to harm” an unpopular group. In United States v. Windsor and Obergefell, the constitutional injury was that same-sex couples were singled out for exclusion or for second-class recognition in the same civil marriage framework available to opposite-sex couples. Unlike those cases, plural-marriage bans do not exclude a specific group from an otherwise open institution—they maintain the dyadic structure for everyone.

Under rational-basis review, the state needs only to show that it reasonably advances a legitimate interest. Several interests align with the mutual-belonging and non-belonging framework and with standard family law administration.

First, maintaining a coherent and predictable marital framework constitutes a clear and legitimate interest. A dyadic structure supports mutual belonging in legal terms: each person is uniquely obligated to, and entitled by, one spouse. This clarity affects support duties, property rules, intestacy, spousal privilege, decision making, survivorship benefits, and dissolution. In Aristotle’s “mutual belonging” framework, the state selects the form of association most similar to a stable friendship of virtue, where two people share a life rather than a series of transactions.

Second, legislatures may reasonably be concerned that recognizing multi-spouse legal status increases the risk of hierarchy, dependency, and coercion, especially when one spouse holds disproportionate economic, social, or religious power. Reducing structural inequality and the risk of domination is essential to Aristotle’s concept of “mutual.” Even in close unions, each person must be valued as an end, not as a means to another’s goals. Aristotle observes that relationships based on utility or pleasure can become fragile or exploitative when circumstances change. In marriage, this warns against treating spouses as instruments for personal gain. From a “mutual-belonging” perspective, a dyadic regime serves as a safeguard by promoting reciprocity between equals and limiting institutionalized domination.

Third, legislatures may believe that dyadic exclusivity better protects each spouse’s dignity as an irreplaceable individual. In Aristotle’s “mutual-belonging model,” marriage is an exclusive reciprocal bond, making each partner uniquely responsible to and protected by the other. The state may view this structure as more conducive to enforceable reciprocity and less likely to create diffuse responsibility, in which accountability among spouses becomes unclear.

Equal protection does not require the state to treat different institutions as identical. Two-person and multi-party marriages are not similarly situated with respect to the state’s purposes in marriage law, such as establishing mutual support, identifying decision-makers, setting inheritance and property rules, and providing a workable dissolution process.

In this context, neutrality means treating similarly situated two-person couples equally within the dyadic institution, not across all possible relationship forms. Windsor and Obergefell addressed unequal access to and treatment within the same dyadic institution, whereas plural marriage seeks recognition of a different form.

Mutual Belonging as a Secular State Interest

Aristotle identifies three bases for love and friendship: utility, pleasure, and virtue. He says that true friendship requires mutuality, reciprocal goodwill, and recognition, stating that “they must be mutually recognized as bearing goodwill and wishing well to each other.” From this perspective, the state has reason to support relationships that embody the highest form of bond. Such relationships are not merely instrumental or transient, but constitute stable commitments in which each partner is valued “for his sake,” with reciprocal duties and mutual care.

The dyadic structure of marriage provides a legal framework for reciprocal, recognized goodwill. Two individuals assume exclusive, enforceable obligations of support and fidelity, organize their lives around a shared unit, and become each other’s partner for significant decisions.

Proponents of plural marriage have raised the Establishment Clause issue to either claim that monogamy limits are impermissibly grounded in religious doctrine, or that the current legal framework selectively favors a Western, Christian view of a two-person model for marriage while effectively marginalizing religious traditions—such as certain Islamic, Hindu, or Fundamentalist Mormon communities—that have historically permitted plural unions.

However, Aristotle’s framework clarifies the underlying justification and demonstrates that “religious roots” do not present a decisive constitutional objection. His account of friendship explains why humans form bonds—utility, pleasure, and virtue—and what distinguishes a fully developed relationship: mutuality, reciprocal goodwill, and commitment to the other “for his sake,” not just for personal benefit. These are clear, secular criteria for evaluating relational forms, and they align with interests the Court already recognizes as legitimate under the substantive due process and equal protection clauses, such as dignity, stability, and responsibility for dependents. A legislature can therefore justify a two-person civil marriage structure in terms of Aristotelian “mutual belonging” without relying on religious authority. The qualities that distinguish a friendship of virtue also make a two-person marriage a rational and secular object of legal protection.

Once the state’s interest is articulated as reciprocity, mutual recognized obligation, and durable responsibility, the issue is no longer whether civil marriage tracks a religious ideal, but whether the state may, for civic reasons, favor a dyadic status that institutionalizes those secular goods. The Establishment Clause does not disqualify laws that overlap with religious teachings or resemble religious moral reasoning. If it did, many legal norms—such as human dignity, equality, protection against exploitation, and duties to dependents—would be suspect, since they are defended in both religious and secular terms. The constitutional issue arises only when the state uses law to coerce religious observance, favor one faith, or enforce doctrinal conformity.

Marriage is the law’s most significant promise concerning private life.

The Court in Griswold v. Connecticut suggested that marriage is a right “older than the Bill of Rights,” describing the institution as follows:

Marriage is a coming together for better or for worse, hopefully enduring, and intimate to the degree of being sacred. It is an association that promotes a way of life, not causes; a harmony in living, not political faiths; a bilateral loyalty, not commercial or social projects. Yet it is an association for as noble a purpose as any involved in our prior decisions.

Each spouse is uniquely accountable to the other in sickness and health, prosperity and hardship, and in decisions that shape the household’s future. This reciprocal, recognized, and exclusive commitment is a public good because it upholds dignity through obligation and freedom within an accountable structure. The state must continue to protect a union that is more than companionship, recognizing it as a legal bond of mutual duty, care, and fidelity.

Joshua Villanueva is JURIST’s Washington, DC Correspondent and an LL.M. candidate in National Security and U.S. Foreign Relations Law at The George Washington University Law School.