Marriages between men and women allow for stable environments for the birthing and rearing of children. Same-sex couples are biologically prevented from participating in the process of childbirth. Furthermore, children raised by same-sex couples are deprived of exposure to either a mother or a father. The Institute on Religion and Public Life asserts: "After decades of experiments with single-parent families, `open marriages,' and easy divorce, the evidence is in and there is today near-unanimous agreement on what should always have been obvious: judged by every index of well-being, there is no more important factor in the lives of children than having a mother and father in the home. Lesbians and gays in same-sex unions cannot be mothers and fathers, except in the poignant simulacrum of pretended sex roles."

Many advocates of heterosexual marriage argue that the legalization of same-sex marriage would serve as a gateway to the acceptance of other nontraditional polygamous and multiple-partner arrangements. Such a shift would undermine the institution of monogamous heterosexual marriage and potentially result in the eventual dissolution of marriage altogether. Some contend that if same-sex marriage were legalized, many couples would wed to reap the financial benefits of marriage without adopting its traditionally held value of monogamy. Commentator Stanley Kurtz, a research fellow at the conservative Hoover Institution forewarns, "Up to now, with all the changes in marriage, the one thing we've been sure of is that marriage means monogamy. Gay marriage will break that connection. It will do this by itself, and by leading to polygamy and polyamory [group marriage]. What lies beyond gay marriage is no marriage at all."

Such advocates are campaigning for the adoption of a marriage amendment to the U.S. Constitution defining marriage as a legal institution reserved for one man and one woman. By ratifying such an amendment, the power to shape the definition of marriage would be taken out of the control of the judicial branch and be placed in the hands of the legislature. In discussing the proposed marriage amendment, the Institute on Religion and Public Life concedes that "the marriage amendment might finally fail, but its passage by Congress and submission to the states for ratification can ensure that `We the People' will not be excluded from the deliberation and decisions that will determine the future of marriage and family, the most necessary of institutions