Recent debates in political theory have seen political liberals advocate and defend a doctrine of restraint, according to which citizens may not rely solely on religious reasons when supporting their favored public policies. This debate notwithstanding, very rarely have social scientists assessed the extent to which citizens actually violate this doctrine. This article evaluates the “political decision-making” model of political liberalism. Data from a nationally representative sample of U.S. adults are used to test this model for legalization of same-sex marriage. Our analyses show that while only a very small percentage of U.S. support this policy solely on the basis of their religious convictions, roughly a quarter oppose it for religious reasons alone. Furthermore, we find that higher levels of religious service attendance and importance of religious faith as well as affiliation with evangelical and black Protestantism significantly increase the likelihood of same-sex marriage opposition entirely on religious grounds.