Y'know... There's this bandwagon status update going around Facebook about the 'institution of marriage'... The religious right would have us believe that the government has to protect the 'sanctity' of the institution.. Which is, at its core, a religious institution. But, y'know what? Congress shall make no law respecting an establishment of religion. Says so in the First Amendment. Will same-sex marriage destroy the institution of marriage? Maybe, maybe not. Doesn't matter. If it's a religious institution, the government has no place controlling it. If it's not a religious institution then the sanctity of marriage arguments fall apart, and the equal protection clause comes stomping in, waving its broadsword and cutting the rest of the arguments to shreds.
Also... prohibiting same sex couples from marrying is discrimination on the basis of gender - not sexual orientation. If two straight men want to marry each other, they are not allowed to - but they're both straight. If a gay man and a gay woman want to marry each other, they're allowed to even though they're both gay. The prohibition is based on the gender of the two parties. Discrimination on the basis of gender is generally illegal.