When a child's biological parents aren't married or monogamous, the child's chances of being abused in some way goes way, way up. There is NO encouraging data on this. Sex makes babies and babies deserve families, which is why all the old stories were about the hero rejecting a love interest who would make a bad parent, and tradition eventually figured out that sex was for marriage. We had it good for so long that we don't remember why and now we're learning the hard way.