homosexuality is a birth defect and that the vast majority of people are exclusively heterosexual is a cultural construct not a biological fact. Homosexuality is the Horny women in Fort Seybert, WV ultimate maladaptive trait ( a trait that makes someone less likely to reproduce). If it were some aberrant it would have died out ago based on biology if not cultural persecution. Some degree of opportunistic bisexuality is a good social strategy for most primates including people. Anyone wanting to weed out homosexuality would have to eliminate all traces of bisexuality which would be a LOT of people. People planning on having daughters could much forget ever being % certain their daughter would never have same sex activity or relationships. Female sexuality is highly fluid and often changes over the course of a person's life. Now some lesbian and bisexual women have been found to have masculinized brains ( shown by finger length, inner ear structures etc.) which could presumably be rooted out if there were tests for the hormonal environment of the womb repeated at two week intervals. However other women who do not have masculinized brains also engage in same sex activity and relationships. This is actually common in women even if it's only sporadic. So daughters are out. Late born sons are also out because hormonal differences make them more likely to be homosexual. So that leaves parents with only the option to weed out homosexuality in their first two sons. It's a polygenic trait so it's going to be hard to root out. It's probably multiple genes interacting in concert with hormonal changes in utero. If parents want to wipe out all the genes for same sex attraction they find them not just in potential but in all potential with any degree of bisexuality. This be half if not more. men are not walking around with a single, odd for same sex attraction. All the men who experience bisexual attraction are walking around with at least a few of the same genes. Why? Because opportunistic bisexuality offers huge advantages to primates in forging alliances, sharing food, defusing aggression and a dozen other things primates do to get along. Add in the fact that bisexual people tend to have more sexual partners and are no less likely to reproduce than straight people.