Mike, that's fine, but anyone can be a polygamist, I was just interested why you single out bisexuals when describing polygamists. And most bisexuals find one person to spend the rest of their life with rather than a desire for more than 1.
Here's my opinion on what this comes down to re: polygamy. Marriage is a bond between 2 people - it's a commitment to spend the rest of their life with someone, and it's a celebration of that. I would argue that if you marry multiple people, it dilutes that bond every time you have another marriage, to which I'd argue what was the point in the first place?
Now, if you're going to argue "You're changing the definition of marriage when you allow 2 men or 2 women to marry, so let's change it to allow multiple people to marry, what's the difference?" - the difference is gay marriage isn't any less of a marriage than straight marriage. It's 2 people making a commitment to each other. If you make it 6 people making a commitment, it's not the same thing.
I'm sure there's holes in that argument. Poke at em.