I have a half brother who is gay. He exhibited signs of being different at a very early age-4 or 5. However, he did not come out until he got a divorce at age 28. He was a very conservative Goldwater man, and boy would we have arguments! He was also a complete jerk until he was about 28. After picking up his PhD. at Columbia he started re-evaluating his life and realized he was a jerk because his life was a lie. Thus the divorce and the move to California where he now edits a gay newspaper. He is also now a Socialist, so we still don't agree politically. (He is a very good looking male as well, which I think plays a part in the genetic and environmental aspects of gay development.)
My view on this is that male gays are born with a strong predisposition to be gay. A substantial portion of female gays become gay because of physical abuse by men, but most of them are also born with a predisposition to be gay in my opinion. Gay behavior is just so strongly evident in the early years, it is hard to argue against a genetic predisposition in most cases.
However, this is still a very controversial and unresolved area. The research to date suggests both genetic and and environmental components play a part.
Several developing issues are quite interesting. More teenagers than ever are declaring themselves bi-sexual, particularly among females. This may not be good news for men, and may influence the number of men who engage in forms of non-heterosexual sex. Also, some therapists are claiming they have successfully turned a gay into a straight. The APA does not claim such therapies are unethical, but lends them little credence. I know many Christian conservatives feel this is a legitimate approach for psychotherapy and needs broader support in the medical communtity.
The research on this topic is very easy to find on the net.
Here's one listing of recent research
I don't understand why anyone should be seriously worried, yet, about gay sexual behavior. Most of the worry seems related to mostly adolescent doubt about our own sexuality. I know as a teen I was very strongly opposed to gays, but that may have been related in part to the gay man who attacked me after a Little League game. I have strong views about aggresive gay males even today, but I have several gay friends who are some of the best people in my life.
Tolerance and love for all of God's children goes a long way to resolve the feelings of conflict we often feel about this issue.