I have no scientific rigor for this, but the possibility for relationships that do not bear offspring makes a ton of evolutionary sense. By including homosexua

I have no scientific rigor for this, but the possibility for relationships that do not bear offspring makes a ton of evolutionary sense. By including homosexuality into a species, you have members who are able to gather and process energy without contributing to population growth. Population growth is a good thing, but too much is detrimental to the species (at least if you don't have access to agriculture). In addition, if the species is a social species, you can have richer communities without the burden of additional offspring, including non-binary genders (as primitive gender roles need not apply to homosexual members). This is especially true for humans, as our offspring is unusually vulnerable for an unusually extended time. Include adoption into the equation, and your species would have a bank of potential parents for abandoned (due to death or otherwise) offspring: in humans, bearing offspring is risky and consumes a lot of energy. Evolutionary speaking, it's not a good thing if all the energy expended in that newborn is left to waste. The possibility of homosexuality is mathematically optimal and it is therefore a deeply natural phenomenon. You don't even need to look at the animal kingdom, it's right there in nature (assuming I'm correct, but I think my argument is compelling).