Oh god, this looks painful. You know, just because girls don't have balls doesn't mean it doesn't hurt... you're kicking them in a bone... I'm assuming this is kicking and not rubbing, of course.
B-ko does this to A-ko in the second OAV of the alternate universe series... also in a couple other shows, that I can't recall. Too tired. Fucking work draining my anime memory banks... Oh yeah this also happens somewhere in the dirtypair series, I believe.
