(which I've never understood why it wasn't spelled "beastiality")

I guess the rationale for specifically bestowing marital rights on same-sex (but non-related) spouses is that there's a significant number of them.

Polygamy, too, there might be a lot of that somewhere in the world, possibly Africa, obscure corners of Utah, certainly pre-modern China when it was normal for a wealthy land owner to have several wives, etc. I suspect it won't be long before polygamy is legalized. Maybe a couple of decades at most.