Married couples in the US most get tax benefits to the extent to which they have a significant imbalance in income, and it seems to be a lot more connected to the idea of traditional division of labor (one partner working in the trade economy while the other provides in-home support for that partner) than children, per se.
They are tax benefits that directly subsidize children, but they aren't conditioned on marriage.
> I find it weird that this argument has so far been completely ignored by either side in the pro/anti-gay marriage discussion (at least as far as I've seen).
The idea that the civil benefits of marriage are inherently tied to the purpose of promoting reproduction are rather central arguments in the anti-same-sex-marriage camp; this was, in fact, the main argument for Prop. 8's constitutionality offered by its proponents.
My apologies then, I haven't followed the gay marriage discussion in general, only the discussion about Brendan Eich.
The injustice is simply in the sit-in-the-back-of-the-bus thing - that a lesbian couple should be stamped 'civil union' just so everybody knows they're not as good. Its Jim Crow all over again.
Disclaimer: I'm single, so I'm not arguing for these benefits. That's just what I always thought the reason was.