Oh, I see what they're trying to pretend they're saying. "Straight couples sometimes have couples by accident; gay couples don't have kids by accident. Having kids on purpose implies that there is a committed, stable relationship ongoing between the people who've decided to have said kids. Therefore, gay couples need no incentive to provide a stable home for their children, because they're already doing so. Straight couples wouldn't necessarily provide that kind of stable home unless you gave them incentives to; therefore, we have marriage, which provides the economic incentives necessary to get straight couples to commit to each other and to their kids."

So very, very, very, VERY many things wrong with that argument. But that's what the're saying.