Sexual exsposure at a young age.

Well regarding propagation as a species, homosexuality is obviously not the candidate. If someone is homosexual and wants to engage in the acts of reproduction which are supposed to produce more copies of both parties involved, but won't because of anatomical limitations, then they won’t reproduce. The pieces of homosexual DNA may eventually fade out.

It could however have a secondary reinforcement effect upon propagation. Based on the idea that homosexual behavior has some type of behavioral foundation in prepubescent sexual exposure. If a person is exposed at an early age to gender-less sexuality then perhaps they have a better chance at spreading their genes around. They may elaborate or "act-out" upon this experience at an earlier age disregarding the appropriate social time frames for the suggested age of sexual experience. Thus getting a "head-start" on their reproductive career. They are engaged in reproductive activities early on. Well before most socially accepted time frames and will likely reproduce successfully due to their learned indiscretion.

In reality though, exclusive homosexuals will always be a minority simply because they cannot create others like themselves. Natural selection will have its way again and again. Unless we evolve into a form of asexual or hermaphroditic reproducing organisms. They will always be viewed as inefficient or "incorrect" based on the fact that they do not contribute to propagation. Though homosexuals may contribute in other ways like adopting an orphan or something.

The point being that those that eventually become homosexual because of their exposure to sex at an early age, may pass on their genes more frequently because of their lack of childlike reverence for sex. Which in turn enables them to engage in sexuality more freely at a younger age.