Wtf! Fucked by the Wrong Guy!!

How brazen can a roommate be? Have you ever brought your girlfriend home, blindfolded her and then disappeared from the room for a short moment to get some toys? This is exactly what happened today, except that my friend’s roommate used my helpless situations to his advantage! How should I best explain the foreign sperm to my partner? Oops, sorry your roommate fucked me while you were away and I didn’t notice?