I think a yes to your first question answers your second. If the subsidy is for producing children, not raising them, then the question is answered. The heterosexual couple provides a service to society that the homosexual couple simply cannot. See the third article for answers to questions about heterosexual infertility, I haven't finished all of the articles but I did read much of that one.
How exactly does collecting a deceased spouse's Social Security benefits subsidize procreation? That doesn't even make sense, in that, one of the spouses is dead by the time it occurs, and having children isn't even a necessary precondition for the benefit. No thinking person could possibly be incentivized to have children by such an offer when all you need to do to collect is marry someone.