We know that sexual reproduction is the main mode of organism behaviors. But in some mammals such as dog and monkey, males will try to the same sex for happiness. Some mammal without mates will even try to masturbate themselves. These behaviors defy the purpose of reproduction, and some explain these phenomena with utilitarianism. I digress.

Homosexuality will NOT exist in an species of rare population. Male monkey will another male monkey (or other animals) simply because he has no mate, i.e. the supply of female is much lower than supply of males. the excess supply of males result in fierce intraspecific competitions for sexual reproduction. So if we claim that homosexuality is for happiness maximization, that would be a wrong interpretation.

In sum, if supply of females are abundant, there would be no homosexuality for male, but it is possible that if excess supply of females exits, homosexuality for females will emerge, just as the female BL fans face.