General

There's really no evidence of what you say, with one minor exception. In fact if it was the fetus being fed with estrogen or testosterone, this would mean the mother would be solely responsable for alternative gender identity/homosexuality.

Many in the gay rights movement, and some scientists believe in a biochemical, if not genetic origin. And it's kind of hard to not believe it plays some role when you look at some of the more effiminate guys and the proportion that are gay and the same for masculine woman. But what ever is eventually found is not going to be the single "gay gene" like exists for sickle cell anemia, or at least if one is found it's not going to explain the majority of LGBT people.

There's also lots of anicidotal evidance it is also choice, and could be a "phase" for some. I've had one cousin go lesbian (as in 100% rejecting men) for 2 years and then decide she prefers men, and another that switched in college and never went back- shes 50 now. Several lesbian friends talk about the number of women who try it in college, only to become a confirmed hetro after they graduate


* There are a couple of rare genetic diseases, such as turner's syndrome that may be likely to cause homsexuality (In the case of Turners, it makes women look less feminine, have lower levels of estrogen, and often infertile, and is associated with lesbianism, but this could just be a correlation.)

I don't have a dog in the fight, but the question is highly politically charged, at least in the US.