I know exactly what you are saying.
This has happened to me several times. They are great people that could be wonderful friends, but the guy does NOTHING for me whatsoever. I wish I could just 'turn it on' whenever I wanted but it just isn't there.
Sometimes I feel like I'm cheating Mr. Naughty out of what could be a fantastic time, but we have agreed that there is to be no more 'taking one for the team' on either side.
Perhaps I need a shrink as well, but I feel your pain girlfriend.