As Lawrence Auster points out, the real problem with same-sex marriage is that it ultimately results in delegitimizing normal marriage. First, the whole terminology is forced to change to accomodate same-sex marriage ("husband" and "wife" are out). While leftists may claim that this is good, because using the same terms for both partners makes the marriage more "equal," (as if the term "wife" must make the woman lesser), the fact of the matter is that gender is a real thing, not a social construct, and there is a lot of damage in trying to force people to ignore their gender.
But the most disturbing thing is that the leftists ultimately want to de-normalize heterosexuality. Heterosexuality is part of the backbone of our society; the heterosexual family unit is what holds us together. When we get to the point where gays demand not just tolerance or acceptance as alternate sexualities, but that heterosexuality ought to be considered an alternative rather than the primary form of sexuality, you begin to witness the breakdown of society.
That's why "heteronormativity" is a good thing, no matter how much resentful people who do not live in the real world dislike it.
That is all.