u/Responsible-Money-15

I'm looking for this manga with this premise : It starts with the bf going to meet his gf. He finds her cheating with what he thinks is a guy. The gf walks out, and the person she cheated with starts lecturing the bf about not being there for his gf and whatnot. Bf notices that the person was actually a girl. Bf has sex with the girl. His gf is on her way back bcz she forgot something in the room and catches them having sex.

It's a pretty short read. Any and all help would be great!