Not sure about this but it should be Renpy game. The game start with a normal cover then when you end the game, the cover changed to girlfriend's back? where she holding(or wearing) condom? filled with cum. The story in the game is about MC dating with his girlfriend but during the date, GF start having sex with a dude. The best scene should be having sex in the toilet and MC tried to see who are having sex but failed and then the GF and dude went to have sex in their house.