I don't think that the only reason two people should marry is to have children.
I'm just saying that the "model" of marriage as one man and one woman is based on the traditional family unit - because only a man and a woman "can" produce children together.
(Sure, there are surrogate parents and test tube babies and gay parents having children with the opposite sex, but in the end, you will always need one man and one woman to produce another person).
I think marriage "laws" don't even mention anything about "love" or feelings. Marriage laws read like business contracts with stipulations regarding children and assets, etc. (Of course, I've only found one online, so I don't know if other states mention "love" in their laws. Still, "love" is not something that can be legislated).
If we're not going to base the legal institution of marriage on that model, then what are we basing it on? If it's going to be based on "love", then we're basing it all on a "feeling". And, if it's based on a feeling, then what would stop the law from extending it to same-sex couples, polygamous marriages, whatever.