The paper argues that same-sex marriage ought to be legalized. The argument is ecumenical and appeals only to basic principles of liberal government. Specifically, the paper argues that if the government is offering an opportunity to one group, then it may not withhold the opportunity from another on the ground that the people receiving it are immoral or that their receipt of the opportunity would spread immoral messages. The only acceptable ground is that the group’s receipt would cause wrongful harm to third-parties that would outweigh the benefits. Same-sex marriage would not do so, and thus it must to be allowed. As part of this argument, the paper addresses the popular stamp-of-approval and defense-of-marriage arguments against same-sex marriage.