This paper offers an argument in favour of the conclusion that it is seriously unjust to exclude same-sex couples from the institution of civil marriage. The argument is based on an interpretation of what the institution of marriage essentially is, and of its essential rationale; the crucial claim is that although marriage is a legal institution, it is also a social institution, involving a "social meaning" -- a body of common knowledge and expectations about marriage that is generally shared throughout (...) society. (shrink)