Objective. The goal of this article is to analyze the relationship between religion, measured in terms of religious affiliation and religiosity, and public opinion about same-sex marriage, civil unions, and a federal constitutional amendment that would prohibit gay marriage. Methods. We use logistic regression with calculated standardized coefficients to analyze data from a nationally representative survey of 1,610 respondents conducted in March–April 2004. Results. Religious variables perform better than demographic measures in models of attitudes about same-sex unions. Non-Protestants are much more likely to support same-sex unions than are Protestants, and individuals with conservative attitudes toward morality and secularism and (to a lesser extent) those who participate actively in religious life are more likely to oppose such unions. On the whole, religious variables play a weaker role in predicting support for a constitutional amendment to prevent gay marriage than they do in predicting attitudes toward same-sex unions. Conclusions. Religious variables play powerful roles in structuring attitudes about same-sex unions. Moreover, homosexuality appears to be a major component of the “moral values” discourse that is currently so popular in American politics. [ABSTRACT FROM AUTHOR]