An excellent observation from ambivablog:
Conservatives always argue the case for traditional marriage in terms of the ultimate purpose of bearing and raising children. Liberals argue that if that were the only rationale for marriage, then infertile or older couples should be forbidden to marry.
There's something unstated there, and it is this: marriage is not only how we take care of children. It's also how we take care of adults. The need for family doesn't vanish when you turn 18, or when your kids fly the nest. We're dependent, social beings, whose joys are more intense and burdens more bearable when shared. Adults can choose to live alone as children can't, yet the great majority still seek to pair up. Times of independence are can be exhilarating interludes, but in the long run, for people to be bonded and stable is healthier for them and for society. That's why the instinct is somewhat independent of childrearing.
Conservatives will rejoinder that gays can still choose heterosexual marriage. That is cruel, and you can say it only if you don't know any actual gay people. If you do, you know that the word "homosexuality" puts too exclusive an emphasis on "sex." What's primary, actually, is who you have deep feelings for, the kind that lead to bonding. Sex is only an expression, except when it's an addiction, and that is a problem that transcends orientation.
It's the heart and the whole being that's oriented. If you're straight, think about how you fall in love. Does it begin in the genitals? Is it all about the genitals? Nonsense. Gay people, same thing. It's a shame that this even needs saying.