Wrong, I'm not assuming every trait must benefit reproductive fitness, they very well could be neutral to it, I am assuming that a trait that has a net NEGATIVE EFFECT on reproductive fitness should fade out of the population over time.
^^^^^^ CB: Yep
^^^^^^
>Understood in this way, there is no puzzle about why exclusive
>homosexuality exists, regardless of reproductive effects: it's not a
>product of natural selection!
But it's quite unlikely that exclusive homosexuality is entirely the result of social pressure. Inheritance studies and twin studies indicate that the population that is remaining exclusively homosexual is genetically influenced. Even if we assume this genetic predisposition somehow spontaneously emerged in the last few hundred years, there is the question of why the trait isn't declining and instead remaining at a steady rate in the population. A net negative effect on reproduction should lead to a decrease in the propagation of the genetic predisposition. This isn't happening, hence the puzzle biologists are legitimately curious about.
^^^^^^ CB: How about some combo of bisexual practicioners and a recessive gene process ?