This happened when I was a freshman in college.
I had reluctantly decided to be roommates with one of my buddies from high school (we can call him Toby) despite him being a bit of an asshole.
I would eventually regret my decision because he was a disrespectful little shit. I won’t go into detail on what he would do but believe me when I tell you he wasn’t a good guy.
One day he had a girl come over late at night (not his girlfriend) and they went in his room and she left early in the morning. I was friends with his girlfriend too so he put me in quite the dilemma by bringing another girl around. I hate when people cheat on their significant other but I really hate it when they expect me to cover for them.
Right then and there, I decided I was going to fuck his girlfriend.
She would regularly stop by our apartment to smoke weed in between her classes on Tuesdays and Thursdays. She had her own key and often when she stopped by I was the only one there. I was fine with this because I enjoyed her company. But now I had a brand new attitude towards our alone time.