In short, the author is arguing:
Treat it as a kind of infertility. When we talk about infertility in couples, we usually don’t assume that both partners are sterile. We just mean that they can’t make a baby together. That’s the way it is for gay couples. We don’t ask infertile straight couples to divorce or break up so they can go have a baby with someone else. We shouldn’t do that to gay couples, either. And we don’t tell an infertile woman that she has to stay unmarried and celibate her whole life. We know that wouldn’t be fair. It would be just as unfair to do that to a woman who was born gay.
Interesting to see an ostensible conservative making the same points that other supporters of same-sex marriage have been making for decades: that being gay isn't a choice or a "lifestyle", that kids aren't harmed just because their parents are the same gender, that gays getting married doesn't somehow detract from "traditional" marriage. Refreshing.
Check out the comment section too, where one objector, eggandsperm, seems to think that technology will soon allow gays to have children with both partners' genes, and that
is much, much more horrible than same-sex marriage...