[Mrs. Don-o]

"... hormones may contribute to, but do not actually determine, the course of sexual orientation in individuals with an abnormal sex steroid history during prenatal life."

I mentioned that the causes ay be multivalent. "Contribute to" can be significant for an individual who is, for other reasons, particularly vulnerable.

Myself, I've never met a gay person who didn't have a deeply unsatisfactory relationship with their same-sex parent. But, again, not everybody who has a crappy relationship with their same-sex parent turns out gay.

There could be a number of intersecting cumulative or contributing factors.

[DJ MacWoW]

From what I've read two leading causes are an overbearing mother with weak or absent father and sexual abuse.

"Chicken hawks" look for young ones that are very vulnerable.