I’ve always been a middle of the road person on the nature/nuture debate.
i believe, a lot like Dom, most people are less than 100% heterosexual or 100% homosexual and that combination of genes and environment affects where on the scale someone is at various points in life.
To me genes and sex are about procreation. Sexual desire, lust, love is emotional/environmental/cultural. So, as humanity becomes more intellectually sophisticated and able to live a more leisurely life, people are more open to the notion that an intimate relationship might not be limited by gender. I think that the sexual revolution that let people start enjoying sex, also opened up the notion that friendships can be as/more intimate than sex.
I mean, how many people my age (over 40) could imagine hearing male actors, musicians on TV, nearly daily talking about their “man crushes” back in 1970s. When i was 20, Liberace and Rock Hudson were suing people for “claiming” they were gay. Roy Cohn was still gaybaiting, while everyone in NYC knew he was gay and had AIDs, etc.
The world is much different these days and the way most americans deal with sexuality and relationships is much improved.