My husband enjoys doing this with me as well - it's one of the few areas where he clearly enjoys domination so I let him do it for ages before letting on that it bothered me more than gave me pleasure. For me at least, it's not pain, it's an acute discomfort of a particular kind, like a severe itch or tickling (not that the sensation is the same -- just that's the level of annoyance vs. enjoyment). Pain is fine. Pleasure is fine. I'm not a big fan of this middle ground. BUT: in so far as I know it gives my husband pleasure I can enjoy the submissiveness of allowing him to continue, at least until it gets totally unbearable! When he gives me breaks, like Ruby says, it is fine, and clearly a turn-on for him (and therefore, me).