I think you've pushed yourself too far outside of the realities of the world.
Yes, on a very simple level it seems obvious that a marriage between two opposite-gendered people is likely to result in children and a marriage between same-sex partners is likely not to.
But you forget about the very significant aspect of choice. Children don't just pop out if the parents are responsible enough -- many married couples choose not to have children. This is perhaps accidental to what marriage is and is expected to be, but the element of choice is just as real in same-sex couple.
Adoption, surrogate mothers, and sperm donors are all real, common options used by gay couples to have children. The biological capacity and drive to procreate tends not to be lessened (I say this only from experience of the gay community) in gay people -- those who want kids -- especially those who find themselves in stable long term relationships -- have kids.
And so, if the state recognizes that a stable two-parent household is a good environment in which to raise children, and that these families ought also to be thought of a building blocks of society, there is very good reason to recognize same-sex marriage.
And for those who choose not to have kids, the gays and straights ought to be in the same boat, married or not.