kopfschuss

My girlfriend and I have sex quite often. We get in the perfect sort of mood, you know foreplay and all that good stuff, then we start to have sex. We go slw at first then faster and harder..like most. Then the problem hits..I come and seeing that we don't use condoms, I have to wipe it of a bit so I dont put in a "wet-stick". And during that process, it becomes limp. After this happened a few times, I became a little worried and asked my girlfriend if she knew what was going on. She of course didn't and that is one of the reasons I am posting today. She had me try sticking it back in right after I had came (she is on BC) and what seemed to happen was it became soft inside her. What is going on? Any helpful advice would be appreciated. Thank you.