It's been tested, and proven.
It's not a choice. It's the way you're born.
Girls who are "gay" may play with cars/trucks as very young children, as opposed to dolls/barbies. Boys may enjoy painting their nails, and dolls, etc. It is not a choice... =]
Also, every male born to a family has a higher chance of being Gay. This does not go for females, though.
Hmm... I found a study, but it's kind of long...
http://www.boston.com/news/globe/magazine/articles/2005/08/14/what_makes_people_gay/
Feel free to read it, if it's THAT important... -_- People won't change their opinions, though. And they WILL continue to argue with... Science :\
Think a bit. Why on EARTH would somebody WANT to be Gay, knowing they will be harassed, discriminated, and critiqued for it? People want happiness, and considering what people think of homosexuality, many Gays don't get too much support. Nobody would choose misery upon themselves. They are merely trying to be who they really are. And there is NOTHING wrong with that :)