Surely, there are even some male only-child homosexuals or bisexuals. There is a gay young man on some chat room claiming to have two older sisters and no brothers. The gene thing might just be part of the equation. The protein thing might be another part. Children are also a product of their environment. Sons abused by mothers in childhood may feel uncomfortable around women. Life experiences may play some part. Sometimes a same-sex partner is just a matter of what is available or handy at the time. I think humans, not inhibited by any social taboos, are largely naturally attracted to whatever looks good to their own eyes or sounds good to their own ears like, for instance, a nice face, sexy hair, pretty white teeth, tan skin, puppy-dog eyes, a smooth, soft voice and a slender body. Most humans, male, female, straight, bi or gay, are averse to the sight of obese or old people.