He may be right in a very narrow sense -- but stipulating that a claim might be a useful predicate for a gentler debate doesn't make the claim correct. My hunch is that most gay marriage supporters disagree with Ross, and that they have more than the kind of anecdotal evidence he cites to back them up.
But even if he were right on the merits, and gay marriage supporters conceded his point, I don't think the tenor and focus of the fight would change much at all. This should go without saying, but gay marriage opponents aren't a homogenous group of elite conservatives whose views about gay marriage stem from quasi-academic beliefs about how the government should promote procreation. There are also outright bigots and bigots who use procreation and similar arguments as stalking horses for uglier views. I'm sure it'd be nice conservatives in Washington DC if gay marriage supporters would just pretend there are no bigots in America. But absent a Supreme Court fiat, the path to establishing gay marriage as a right everywhere in the country means contending with all of its opponents, not just the polite ones.