I've been looking for this manga for a while now. It's actually a tankoubon and I only know one part of it. A guy and a girl are doing the nasty at home, only they're doing anal. Apparently the guy was into this kancho prank, in their younger days, and pulled it on her, which clearly awoken her anal fetish. However the guy wants to try it the normal way and the girl is fully enthusiastic about it, until they get to the hospital to check her hymen, when she starts getting cold feet. There's even a funny scene with the guy being calm when the girl tells him what the doctor says about her condition.