This is simple.
There is a difference between homosexual behavior and homosexual people. The difference is that gay people are ATTRACTED to the same sex. As pointed out earler in several post, one can have gay sex and not be gay, but one cannot be attracted to the same sex and not be gay, even if they never engage in sexual activity. It is about perference, not the actual activity of sex. Your either perfer the opposite sex, the same, or both,.....straight, gay, bi.
Now, about it being a choice. Being gay IS NOT A CHOICE, for those that are truly gay. Bi-sexual people and dumb people have confused the issue, some people think they are gay or want to be gay but are not. Sexual perference and attraction are not things that somebody choose. These are things that are developed before one is born thru DNA, and modified throughtout life by the eniviroment and culture that one lives. But nobody choses who they are attracted to, it is a biochemical response to favorable stimuli.
If you disagree with the idea that people are not born gay, then ask your self a few questions: At what age did you DECIDE you were striaght? What made you decide you wanted to be straight? How long did you ponder this before making a decision? When did you made your decision? Did you tell your friends? Was it a final decision etc...? Get the point? You never answered any of these question because you never had too, you just knew and did. There was no choice.