The abstract from an academic defense of marriage discrimination:
In the article, we argue that as a moral reality, marriage is the union of a man and a woman who make a permanent and exclusive commitment to each other of the type that is naturally fulfilled by bearing and rearing children together, and renewed by acts that constitute the behavioral part of the process of reproduction.
Marriage is defined by exclusivity—it's defined by monogamy—but only when gay people want to get married. Straight people don't have to be married to be monogamous or monogamous to be married.
Marriage is permanent—it's for life—but only when gay people want to get married. Straight people have been marrying and divorcing and remarrying and redivorcing for centuries.
Marriage is about bearing and rearing children together—not the turn of phrase I would've chosen—but only when gay people want to get married. Straight people can be married without rearing children and rear children without being married.
Marriage is—and I'm just going to quote this bit—"renewed by acts that constitute the behavioral part of the process of reproduction." In other words, straight sex is magic—it makes babies!—and straight sex that doesn't or can't make babies ("In the butt, Bob") is still magic because it "renews" the marital bond, i.e. it reminds you why you married that person in the first place ("She's really been on my nerves lately, but... man... she gives excellent head").
Sorry, academic bigots, but your argument is bullshit. Straight people long ago redefined marriage and no rational argument can be made for excluding loving same-sex couples from the institution of marriage as redefined and currently practiced by straight people. Marriage is the legal union of two equal and consenting adults. Period. A marriage can be monogamous or not, it can involve child rearing or not, it can be for life or not, it can have a religious dimension or not, and it can be sexless and/or very infrequently renewed or not.
Opponents of same-sex marriage should just level with us: they don't just believe that same-sex marriages are immoral. They believe that most opposite-sex marriages are immoral. It's not marriage equality they have a problem with but modern marriage itself.