I believe it is about maintaining more than privilege. It is about maintaining prejudice. Enshrining prejudice in the law gives it unjustified official support. Not long ago, anti-gay people could say being gay is wrong because it is illegal. Once laws that accommodate prejudice are removed, there is no longer any official support for the irrational prejudice.<quoted text>
Of course we should allow civil marriage for same-sex couples. It's in the best interest of the couples, any children they may have, and society as a whole.
But the anti-gay aren't interested in the best interest of society.
They are only interested in protecting and maintaining their own privilege.
Because there is no scientific basis for it, that leaves those who promote it with nothing more than a few questionable religious texts which are contradicted by other more important texts, and not agreed upon by all religious scholars, leaders, and believers. They need that official government recognition to support it.
The tragedy is, as you know, the resulting dehumanization and stigmatization results in needless suffering and death in a wide variety of ways. Laws like DOMA perpetuate the deadly prejudice they were designed to indulge.