Same gender marriage is different than gay marriage. I want to reserve the distinction, for an option most people haven't yet contemplated.
Gay marriage is Bill and Debra are man and wife, yet both are gay. I used to um, not oppose this, but find it silly. Sometimes it is.

Gay marriage has been going on, it's a legal option for gays to marry opposite gender gays, have kids, etc., and they do. Nobody cares much about the bedroom, it would seem, as long as inter-gender marriage is at least parodied.

The new option in same gender marriage is that heterosexuals might decide there are benefits to being married, a la Lennon and McCartney, (who later divorced). One can imagine party bachelor hetero dudes in pairs now, best-friend girly-women in sets--perhaps in part as a force-field to protect them from the dangers of cross-gender marriage.

It may erode traditional relationships, in that people may begin to realise they can form committed relationships to anyone they want, without any permission at all. The American norm of huddled alienated clusters, fearful and mistrustful of everyone else, might begin to degrade. No wonder conservative fascists are concerned. They are right to oppose it--so far as preserving their way of life is potentially threatened.