Sounds like you're in the friend zone with the one you really want. Sucks for you. Even when/if she breaks up with her current bf you can be damn sure she won't want to "ruin what [she] has with you".
I'm also not one of those "sex positive" people who thinks sex just for the sake of sex is an okay thing. I don't really agree with it... but I also don't like football and most of America disagrees with me on that too and we all manage to co-exist anyway.
Personally, I recommend you tell the young one it'll never happen. Then tell the other one whether or not you can stand being without her. If you can't, tell her you have to be with her or not at all. Girls just don't seem to get that a guy can't be "just friends" with a girl he's into. Even if he says he's okay with it... he's DEFINITELY not. All they're doing is hurting him.
Seriously girls, if a dude likes you... either go out with him or don't be around him. The last thing he'll ever want to hear about is your problems with your current bf, your crushes (that aren't him) or help you pick out an outfit for a date. If he won't tell you that, it just means he has no balls. Doesn't make it not cruel to put him through that crap.