The Chimera Hypothesis: Homosexuality and Plural Pregnancy makes the surprising claim that:

at least 50-70% of healthy adults are chimeric to some extent.

The hypothesis can explain some homosexual and transgender tendencies, and suggests some reproductive advantages that offset those reproductive disadvantages:

Therefore, women prone to having more than one egg fertilized, but whose pregnancies only resulted in only one live birth, would have the optimum level of fertility. A side effect of this could be an increased incidence of chimerism in human children … even a relatively small increase in female fertility, which is really the limiting factor in human population growth, could outweigh the disadvantage of less fertility in a small number of male infants.

One observation that this hypothesis doesn’t explain is why there are many more homosexuals than transgenders. So my guess is that it explains only a modest fraction of the homosexuality that we observe, but might explain the observed frequency of transgenders fairly well.