As JRochelle pointed out upthread animals are not able to give consent. Someone brings this up in every homo-marriage thread and it is an argument that simply does not hold water. Personally I could care less if some want to practice polygamy, it's not for me but then again no one's forcing me into it either. Plenty of historical precedent, much more so than for gay marriage.
Exactly. I also have a problem with something being illegal when it is formalised by the state (ie applying for recognition of their union) but fine if done informally (ie living together as a trio).
That said, I don't think groups opposed to such unions should have to recognise them but that only reawakens the old argument of whether the state should have any part in recgnising marriage or not. If it does we're always going to have someone 'pushing the envelope'...
Mind you, my real beef with polygamy is that nowadays (This case excepted) it always seems to involve young kids below the age of condsent being pressured into rleationships with sleazy old men with the kids own families helping to apply the pressure.
I could care less what consenting adults do (As long as it's not in my face) but some calls of polygamy are no more than a mask for abuse.