Can you offer a mechanism by which doubt and ambivalence "screw people up psychologically"?
Can we at least agree that the physical act of sticking Y into X doesn't somehow change neuron connections, resulting in the claimed psychological damage from sex at a young age?
So then we're left with the question of where the psychological damage comes from. Strings' argument seems to boil down to some inherent psychological damage caused by sex at early post-pubescent stages in development, caused by genetics and mediated by chemical signaling (expressed as emotions). Unless you or he are suggesting that there's some rational mechanism for self-caused psychological damage based on rational processing of the experience of sex, which I find equally bizarre.
People regularly go insane all the time based on ideas. It's been demonstrated fairly conclusively that psychologists can convince people, particularly young people, of things that aren't true. They then (re)act to those recollections as if they were true. Is it such a stretch to conclude that societal taboos that seem harmless enough can convince people that something they did was wrong, maybe leading them to the subconscious conclusion that they're social misfits, inducing all sorts of other unhealthy behavior?
I can envision that even consensual sex between an older relative and a young teen (or child) could be developmentally damaging due to complex changes in family interactions. However, I can't see how sex with peers or random other people would inherently cause problems in social or psychological development.
I find that people have urges to do many things that are considered wrong. Should such taboos be dropped, to avoid psychological damage?
There aren't very many other taboos that involve consenting individuals and involve activities conducted in private. And although I'm sure you'll pull some tangential taboo out of your hat, I'm pretty sure it won't involve an activity that is a biological/evolutionary necessity at the species level.
Why do you think the taboo is damaging, rather than the sexual activity?
Because it's a common pattern among higher mammals, and our closest genetic relatives, that they have sex starting at (or even slightly before) puberty. Any "harm" this causes is pretty obviously due to some higher (psychological) phenomenon that is not primarily genetic/chemical.