I believe that a girl born after a older brother has a higher chance of being a tom boy because of the antibodies left in the mother's body. The same thing might affect men born after an older sister. But I think a lot of homosexuality is born of lack of success with the opposite sex and a familiarity with the same sex. Clearly more is involved including earlier experiences. I go with the continuum thing, everyone is on a line from hetero to homo and where you are sometimes gives one side or the other a chance to pull you in. Not genetic, but nevertheless inherent.
This does not mean I want homosexuality to be normalized, on the contrary, I would like to see it totally underground and given as many chances to disappear as possible.
There are those who propose that it is population-pressure driven.