This doesn't make any sense to me given the context of his post. His point was that the government is offering incentives to couples who can have children, because without couples having children society does not progress at all, regardless of what else they do.Of course a society wishes to continue, but it also seeks to progress. The two things are not mutually exclusive, but then they are not always conducive to one and other. Certainly there is no obligation for a society to be transfixed by only doing things that are focused solely on its continuance - and in light of this your argument doesn't hold up.
It is silly to think that allowing gay marriage would just suddenly eliminate heterosexual marriages, but there are already plenty of straight couples that don't have kids as it is I'm not sure how solid either side of this argument is.
Personally, a more serious issue with the way that legal marriages are performed right now is, if gay marriage were just suddenly legalized, churches that do not believe that homosexuals should marry would be forced to marry them if they were asked to, but this is more a matter of laws needing to be changed to accommodate such a thing, if it ever happened.
Finally, the reason why no President in the US will ever agree to having gay marriage legalized is because they would lose the entire bible belt by making that submission. Personally, I think the biggest issue surrounding gay marriage has nothing to do with marriage at all and has everything to do with civil rights like next of kin, hospital visitation and insurance privileges. Any adult ought to be able to provide these for any other adult dependent in their care, regardless of age or gender.