Proponents of same-sex marriage often defend their view by appealing to the concept of justice. But a significant argument from justice against same-sex marriage can be made also, as follows. Heterosexual union has special social value because it is the indispensable means by which humans come into existence. What has special social value deserves special recognition and sanction. Civil ordinances that recognize same-sex marriage as comparable to heterosexual marriage constitute a rejection of the special social value of heterosexual unions, and to deny such special social value is unjust.