Sexuality is so complex I doubt we will ever know what makes someone attracted to the same sex,from the many other bi and lesbian women I've spoke to some say they knew from a very young age they were gay or bi, others in their teens, some never had a clue until adulthood or feel they turned completely having been boy crazy to nothing at all, some looking back can recall signs they missed others can't explain it at all. If there is a gene then would it even occur in bisexuals or those who realise later in life that they have same sex attraction, and what triggers it to go off later in some than others.
I do think there is a lot less purely heterosexuals than we are led to believe, women at least are encouraged all their lives to be emotionally And physically close, to compliment each other, have girl crushes be intimate for laughs. Friendship, admiration, attraction can all be blurred sometimes it takes a long time to separate them and spot the trigger.