Pedophilia is probably next, to be followed by polygamy ... bestiality is still a bit of a stretch.
Actually, polygyny is permitted by Divine Law. The only law against it is civil law, which is of course to be followed so long as it is the law. And the idea that polygyny is permitted doesn't mean it's for everybody. There are all sorts of considerations such as can the husband provide equal time, affection, and sustenance for each wife, etc.
Polyandry, on the other hand, is strictly forbidden and cannot be legalized by any state. Perhaps that is the tack the libs will take.