Wow, very nice synopsis, Cainad! I'm not sure where we go from here, honestly. I tell men and boys the same thing, consistently, which is that if you want to eventually have a relationship with a woman, start by making friends with women. Real, actual, care-about-their-feelings-and-experiences friends, without sexual expectations.
It's a good start, and ultimately that's the exact message that needs to get through.
I think it's the "without sexual expectations" part that is the most thorny. Sure, teenage boys and young men can be horny and disappointed that they aren't getting any, but more central to the problem is the idea that being turned down represents a failure
to them. Since they usually don't want to internalize the idea that they "failed" to get laid, they lay blame on the girl/woman for being too selfish to see what a great and fuckable guy they are.
The cognitive dissonance, of course, is pretty easy to digest when you're both horny and trying to assuage a completely unjustified sense of failure. I think it's important to bring it to light and emphasize that "being a nice friend" and "demanding sex" are incompatible ideas.