I am in favor of civil unions, as I argued two days ago. I favor them despite the fact that I think they will contribute to undermining marriage. I have been asked by some respondents to explain why I think civil unions would harm marriage at all.
Some argue that if I am free to marry, what difference does it make if other people conduct their relations differently? I believe that we should tolerate many different ways of conducting our relationships. But this is not because they don't affect mine.
Marriage is a social institution. It works because society as a whole believes in it, and supports marriage as such. My marriage is not just made by my wife and me, but by the whole society. Marriage is one of the most powerful of social institutions because it has huge social support.
If marriage were treated as just another private contract (as it is in increasing danger of descending to), fewer marriages would be formed, more marriages would fail, and each marriage would be more tenuous because less taken for granted.
Marriage is the complementary union of a man and a woman to make and raise children. This is the social model for marriage, around which all actual marriages are arranged.
Civil unions are, I think, a species of intense friendship between people of the same type. A civil union is designed to benefit the couple. Children may be brought into the union, but they are secondary.
If some people pursue unions with their opposite sex partners to benefit just themselves, with only a secondary consideration for children, this undermines marriage. Not just that marriage, but marriage as an institution.
It is natural in an individualistic culture such as ours to recast social institutions to benefit me, rather than forcing me to discipline and sacrifice myself for others. Even without children, marriage entails the discipline of submission – mutual submission. With children, to that submission must be added self-sacrifice. It is hard for us to sustain this high standard of marriage anyway. We only make the task harder when we add a competing marriage-like relationship that does not entail the same kind of self-sacrifice, and may not even require self-submission.