Well, I guess we disagree about the definition of marriage. Or at least part of it. Committing to a stable, monogomous partnership is enough for me. Children are certainly not required (and I say that as the happily married father of one child, and hopefully more in the future).
Also, two follow-up questions: First, should opposite-sex couples be allowed to marry if they do not intend to have children? Second, internationally, there are far more children in need of adoptive parents than there are opposite-sex couples looking to adopt; do you think that these "extra" children would be better off raised by married same-sex couples or in group homes?
Marriage is an institution that is designed to create the ideal environment for raising children, you do not require children to be married.
Personally, I'm against international adoption (in general) at the moment because it is not particularly ethical; essentially, a large portion of children that are available for international adoption are being sold to wealthy westerners, and I have heard it suggested that this is against their parents will. I believe the correct approach to deal with "unwanted" children in developing nations is to increase the standard of living, and to encourage the use of birth control; whether it is artificial birth control (the pill/condoms) or more natural methods (the rhythm method). I don't necessarily have a problem with homosexual adoption, but with there being a massive shortage of children available for adoption I think we should be able to place them in the "best" environment possible; which could potentially discriminate against people for countless reasons. As an example, you might want to place a baby with a family that most closely matches their ethnic background and, since there are far more black babies than white babies available for adoption, the waiting list for black adoptive parents might be shorter than white parents. Of course, this would not preclude white parents from being given a black baby; after all, any loving parents (including parents of different ethnic backgrounds, or homosexual parents) are better than being a ward of the state.