My 2 cents:
The reason why the state is even involved in marriage is because marriage produces goods that the state is interested in. These goods being kids. To incentive this they give married couples benefits. And here is where my problem comes when people argue that gays shouldn´t get benefits, because they can´t have kids.
A. How many straight couples don´t have kids?
B. If you would allow adoption for gay couples, it would improve the life of a orphan considerably
C. How many parents have kids and raise them poorly?
In the end marriage is just a word, and if you want to label yourself as being married go ahead to do so. The state should give gay couples the same benefits as straight couples, because on average they raise a child just as well (according to recent studies it´s better, but the data might be biased).
Same sex couples have been allowed to get civil unions in America for a long time. Those provided most of the same benefits as marriage, such as the ability to file taxes jointly. I don't think the issue of child production is really a factor here. Married couples that don't produce children have the same marriage license as those with children.