If individuals have the right to choose their spouse, and if spouses are equal partners without different gender-defined roles, then gay couples have the right to marry. Many today -- including the Mormons -- do believe men and women have distinct roles in marriage, and therefore don't believe gay couples can marry, but as far as our society is concerned, these changes have already happened. Gender equality is a legal and social reality now, and so, therefore, is gay marriage.
There's a great editorial today making this point in the Washington Post:
Gay marriage isn't revolutionary. It's just next.