Summary:
Guy takes Girl home from bar.
Guy gets some, then goes to the bathroom.
Guy’s friend comes into bedroom and has sex with Girl, she thinking it is Guy and not Guy’s friend.
Guy comes back from bathroom, seeing friend having sex with girl, goes to sleep on the couch downstairs.
Girl figures she was duped and Guy’s friend is on trial for sexual assault (quite rightly, I suggest)
I think I saw something like this on Three’s Company once. Or twice. Or every episode.