choosing to be whatever (lgbt)
Observation: there's strong evidence it's genetic.
No, epigenetic. The statistical incidence of male homosexuality goes up, the more male child births preceeded the male in question's birth (from the same mother).
Meaning, if you are an only child, and male-- not likely to be gay.
If you are the 17th child of all boys, you are quite likely to be gay.
Notably, it's not social. Having many older adopted brothers does not affect the chances whatsoever. Only biological brothers, and it even holds if they're raised apart. So it holds biologically, but not socially.
From a population dynamics point of view - this could actually be
beneficial to the group. If there is a large excess of males born, then more of them would be gay, thus reducing competitive pressure for the females. Since we live in a social species those excess males are still contributing to group survival. So this is something that could actually have evolved for, since the younger brothers won't be competing with the older brothers in this case, making the original mother's dna line more likely to survive. This answers a problem with "gay genes": if a gene stops you breeding why doesn't it just die out? The clear answer is that the gene only comes into play in specific circumstances. After all, there are many species where only a small number of individuals breed, and their population dynamics are clearly understood to be inherited.
There is also strong evidence that females of all mammal species can rig the gender of their offspring based on environmental needs. If there is abundant food supply, then having a male child can theoretically produce more offspring (since they will be strong and can mate with multiple females), and if food supply is low, then having a female child is a better guarantee of offspring. They've done some studies to show that this seems to be the case.