Personally, I don't have a problem with polygamy or polyamory either, or the concept of having muitiple husbands. I expect the flame police to descend upon that statement anyday now. I can sum up the gay marriage/civil union argument like this: the government (not churches) give out heterosexual marriages and call them marriage. If, in fact, everyone's equal under the LAW of the United States, then the point follows that the government should recongize gay unions as well between two consenting adults of the same species of humanity, and the legal rights of such partnerships there of. Note the words 'adult' and 'species'. I don't condone beastality, considering forcing yourself on an animal is a rather disguisting version of rape, or pedophilia, which is dealing with a developing child.
The hitch is that the majority of arguments cast against this come from a religious document, not a secular text of law: the Bible. Some groups of religious conservatives say (their favorite book to cite: Levictius, which also bans polyester/cotton blend clothing) since that the Bible deems homosexuality wrong (and polygamy as well), it should be wrong considering we will all rot in hell as a country if we allow this. Personally, I find that argument amusing and sad considering the AIDS epidemic during the 80's came out of the homosexual community because they were the most prone to have muitiple partners, considering the government didn't advocate homosexuals forming partnerships (anti-gay marriage was on the books and still is to this day). It's the same old argument of church meddling with law, a thing we've witnessed since the whole 'church vs. state' thing, which makes no sense considering we're supposed to advocate a number of religious beliefs here in the US.