Gay people will form couples marriage or no marriage, so why not let them marry? I mean, we no longer consider it okay to own slaves even though the bible says it's okay as long as they are from another country. We no longer feel the need kill people working on the sabbath. We no longer feel the need to gather the whole village to stone people to death for wearing garments of two different fabrics. Also, eating shellfish is no longer considered an abomination, so why is homosexuality considered one?
So, why not just embrace the message of loving your next and get rid of the hate? That is after all the main message that Jesus teached, to love one another.
Oh, and by the record: People are not born with virtues, but rather we inherit them mostly by the ones that are close to use when we grow up (usually our parents). And don't start saying something that homosexuals lack virtue, rather it is the one that decide to hate and oppress that lack virtue.
As for polygamy weddings I say why not? The western society is the odd one out living in an individualistic society while the majority of the world live in a collectivistik society. And no, letting couples getting married, as is already allowed, won't open any real door for poly-weddings. The only change is from "almost all couples may marry" to "all couples may marry". What is strange about that?
And a side note on unnatural things: Many christians tend to say "homosexuality is not natural" but when proven that it is indeed very natural you come up with the lame excuse "but humans aren't animals". But thing is: If homosexuality exists amongst humans, that must be natural, right? It's like how it's natural that some have blond hair and a few have red. It's natural to be left handed. Just because it's not in the common norm doesn't make it unnatural, it only makes it unusual. See the difference.