>>936882069 (OP)In women? Primarily circumstance. Women are mostly attention seekers by nature and they will gravitate to whomever will give it to them at the end of the day. Ugly, abused, oppressed, or simply lack of a good male role model all greatly increase odds of homosexuality in women. The more of these descriptors they have, the chances they will be "gay" start going exponential.
Men are similar. Just add in degenerate horniness as a huge piece of the puzzle and tou pretty much have the whole picture.