And by this logic if the priority is the child must always have both a mother and father above all else, they must outlaw divorce and out of wedlock motherhood, meaning they'd have to not just allow, but force abortions in cases of rape or premarital sex. In fact, they might have to end the child's life if a parent passes away.

How far to take this ideal is the question. Even if they believe a child is better off with two cis-het parents, does that ideal override all other rights and concerns?

And that's not even addressing that marriage is about way more than children. They need to provide a 100% equal option for everyone else, not the "separate but equal" thing that domestic partnerships provide which was never even close to equal legally.