Originally Posted by Pip

I'm pretty much asexual. I masturbate frequently, but I don't really want sex with a partner. I've tried a few times - didn't feel a thing. It was weird actually, having someone touch you on all your good spots and feeling NOTHING. So, um, I'm probably asexual. Or haven't met the right one yet.

So at least in my case I think it is brain chemistry. The wiring is there and all the parts are working, but the system doesn't respond to external users.

From an evolution perspective, I think it's the same argument as for homosexuals. A non-breeding adult in the close family is pretty much an extra parent/breadwinner.