Ehhh this irks me... It's not a choice. And I know you disagree, but because it's not a choice, calling a legal union of a same-sex couple anything but marriage results in a separate-but-equal institution, which we've already decided isn't okay.
This may not be the best place to have a discussion like this, but by "choice" I don't mean that the attraction is a choice, I simply mean what you do with your feelings is a choice. For example most heterosexual guys have a natural bend for pornography, but whether they view it or not is their choice. This is a purely sexual example and is very incomplete because feelings for other people involve more than sexual attraction, but it's late and I can't think of something better.