Without the moral baggage of WW2 and the unfortunate implementation history of eugenics programs in the 20th century, it may be easier for people to accept that encouraging the gradual improvement of the gene pool would actually be a positive thing for human civilization.
The one and only semi-acceptable eugenics practice in the modern West is the termination of pregnancies of Trisomy-21 fetuses. Any hint of extending that acceptance any further, and you're instantly a Nazi.
Moreover, genes are not binary in the sense that there are "good" or "bad" traits. It depends on the time and place like sickle cell anemia, which gives you immunity to malaria, but has the obvious downside of making you anemic.
And there are those naturally immune to diseases like smallpox or even AIDS , even diseases we have no knowledge of right now.
I once had a fellow coworker who hinted at the fact that he was pro-eugenics (semi-jokingly) as he was a varsity athlete from a good college in good health. I had to remind him that just a bit over 100 years ago the US signed the Chinese Exclusion Act and that Hitler would not have considered him to be anything close to a true Aryan.
So one person's improvement might be another's worsening. And we've seen our opinions evolve over time.
As a consequence there's a serious ethical concern. As flawed human beings I don't think any of us can say with certainty how we should manipulate the gene pool. Allowing Nature to do it herself has worked reasonably well thus far, if you consider the evolution of Homo Sapiens to be a positive outcome.
I don't wish to come across as callous, but wouldn't homosexuality self-eliminate itself from the gene pool nowadays because there is no societal pressure anymore on homosexuals to maintain appearance of heterosexual couples?
Of course, if I'm correct in my assumptions, it's fascinating why homosexuality exists.
That slope isn't anywhere near as slippery as you're trying to make it out to be.
The gradual improvement of the gene pool will likely happen anyway, as parents choose to use new technologies to improve their future children in various ways.