I seem to remember that the youngest sons in families with more than one son are more likely to be gay.

Okay, looks like I remember right. Of course no idea how valid those studies are, and, well, correlation is not causation, but this seems to make at least some sense.

https://www.newscientist.com/article/2156010-we-may-know-why-younger-brothers-are-more-likely-to-be-gay/

"The team collected blood from 142 women, and screened it for antibodies to a particular brain protein that is only made in males. They thought this would be a good candidate, because it plays an important role in how neurons communicate with each other, and because it is produced on the surface of brain cells, making it relatively easy for antibodies to find and detect it.

They found that the mothers of gay sons with older brothers had the highest levels of antibodies against this protein, followed by the mothers of gay sons with no older brothers. Women who had straight sons had less of these antibodies, while women with no sons had the least."

Seems to be called "The big brother effect".

https://www.newscientist.com/article/mg17723884-900-the-big-brother-effect/

This is of course something that would be liked well enough these days because it makes homosexuality look quite natural, and something you can't do anything about unless you start to advocate that women will not have many boys. But frankly, it actually would make at least some sense biologically too, when you think in terms of the relatively small hunter-gatherer bands - fewer men competing for the women, some of them turned into kind of worker bees that would still benefit the group, the same way women past menopause are presumed to do even if they can no longer have babies.