I've written a bunch of articles this week for The New Republic. Please read them!
This is a big deal because one of the reasons he promotes monogamish for straights is as a way to have couples stay together for the sake of the kids, even if doing so means having a companionate marriage and a piece (or several pieces) on the side. But the fact that someone isn't a primary partner doesn't mean they can't, in turn, bring about a pregnancy. It's not, of course, that a child born to such an arrangement is doomed for life, but even by Savage's own framework, it's a non-optimal situation.