Abortions motivated by a fetus's perceived homosexuality.

[DetlefK]

136. Genetic proliferation depends on the social feedback.

Humans are shallow. If someone is pretty on the outside, then he must have good genes, which means, you really should have sex with that person.

Down-syndrome changes the phenotypical appearance in a way that moves an individual farther away from the parameter-set that defines beauty. Therefore, people with down-syndrome are less likely to procreate.

Homosexuality, on the other hand, is not visible by the mere eye.
Even if the male embryo inherits the appropriate genes from the mother, he will not automatically turn out gay. (Otherwise every son of a particular mother would have to be gay.) This means, there are men and women out there, who carry the genes but aren't homosexual.
(For female embryos: From what I have read so far, lesbianism isn't genetic but determined by the chemicals that influence the embryo in utero during the brain-formation-stage.)

My point is, that the homosexual genes were advantageous and undetectable in the past, so they must have spread far and wide.
When they become detectable in the future, getting rid of them could only be achieved by keeping a sizable (or maybe even overwhelming) part of the population from having children at all.
And even if a society attempts to go there, all of the lacking population would have to be born by mothers which lack the fertility-bonus of the homosexual genes.