Damon Linker over at The Week has been superb lately. Following is a point that’s been made by others as well, but cannot be stressed enough:
For many advocates of gay marriage, liberalism is a holistic, comprehensive ideology with its own distinctive vision of the human good. This vision advocates the autonomy of individuals from received traditions, and their liberation from constraints both external (political, social, cultural, religious) and internal (psychological), which it invariably treats as forms of oppression. Liberalism in this sense denies that sexuality should be subject to moral judgment or evaluation of any kind, beyond the consent of the parties involved in the sexual act. As long consent has been given, anything goes.
In addition to holding out this ideal of individual autonomy, comprehensive liberalism demands that each individual’s choice of how to live be recognized and positively affirmed by everyone else, no matter what it involves (as long as it doesn’t infringe on anyone else’s equally free lifestyle choice). Comprehensive liberals also tend to treat the refusal to grant this recognition and affirmation as an act of illiberalism that ought not be tolerated. Many go so far as to think that liberal governments should force the recalcitrant to comply with the liberal ideal, at least in any area of life that can plausibly be described as public.
Liberalism in this sense is a zero-sum game, since its advance through public life often requires the equal and opposite retreat of non-liberal visions of the good.