I read something a while back and can't find it now. If I'm remembering right, it's about a guy in the bathroom at his girlfriend's house, and his girlfriend's little sister runs in to use the bathroom and ends up sitting on his lap, then obviously sex ensues. I searched "bathroom intercourse" here, but couldn't find it.

Hajirai Break by Homunculus, I believe the one-shot is bathtub crisis and the sequel is the fitting crisis. If you expect another story, I wouldn't get my hopes up.