Congenital adrenal hyperplasia – a group of genetic disorders – appears to increase rates of same-sex attraction in women, while possibly decreasing its incidence in men: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7082355/ (both findings make sense if we view it as a biological promoter of gynephilia, sexual attraction to women)
I think it is likely a complex mixture of both biological and social causes, and the nature:nurture ratio likely varies between individual cases.
Selective pressures may alter the incidence of some of those biological causes over time, although there is no guarantee. Whatever social causative factors exist may also vary in incidence over time, in response to societal changes.
The other factor to consider, is that with continuing advances in medical technology, it may be possible to interfere with some of these biological causal pathways: for example, if it is true that maternal anti-NLGN4Y antibodies are a major cause of having same-sex attracted sons, then if a test for those antibodies became widely available, it might influence the fertility decisions of some women; eventually, some kind of "anti-antibody" might be developed, so a woman who wished to reduce her odds of bearing same-sex attracted sons could take it to reduce her own anti-NLGN4Y levels.
Beware
While there is no [identified] gene for gaiety, behaviour in general is a constellation of traits.
For instance, the tendency to alcoholism is hereditary. https://www.niaaa.nih.gov/alcohols-effects-health/alcohol-us...