You know, while I've had some big struggles with homosexuality and Christianity, I've never once thought polygamy was a sin.
Lots of people in the Bible had multiple wives, and there wasn't anything specifically stating it~
I think what Jesus was saying in that passage was "Yeah, you aren't married, but you've been with 5 men, and you aren't married to the one you are now. I know who you are, and what you've been up to, but I want to give you life."
I personally don't believe he was condemning nor condoning polygamy.
He's focusing on the big picture, life! =D