Haven't really heard anyone use "get bent" for about a hundred years now; I'm pretty sure it did just mean "get F**ked"; I'm also pretty sure that no one I ever knew could give you any logical reason why the idiom developed. You're the first person I've ever known to think about it.
Never heard anyone say "I've got nine". My intuitive guess would be that it's a grandious claim to have "nine inches", but who knows if it's really that close to logical?