Perhaps because it (or they) would affect males and females differently? Simple hypothesis: there is a gene which will cause people of either sex to be unusually strongly attracted to males, and another which causes people to be unusually attracted to females. The former gene would tend to cause males carrying it to lean toward homosexuality, but females carrying it to nymphomania; the latter gene would have opposite functions. While homosexuality may not promote dissemination of one's genes, nymphomania most likely would.
I have no idea whether the hypothesis is correct or not, but it suggests that a genetic predisposition to homosexuality could be an evolved trait. Of course, societal factors would probably play a much larger role.
The FACT is that the pathology of same-sex attraction (there really is no such thing as "homosexuality") is actually well-known, well-documented, but patently ignored due to political correctness.
Look at the NARTH website; there's a lot of good, reasonable and rational information there.
Same-sex attraction occurs due to neuroses that cause these individuals to develop distorted views in the proper relationships with members of their own sex.
That's why Reparative Therapy works: It strives to "reparent" the individual who is thus affected, and develop healthy attitudes about relations with both their own the and the opposite gender.
Again, the pathology was simply removed in the 1970s by the APA due to political correctness; the pathology remains.