As usual, Finn had been having trouble with Flame Princess, so Marceline decided to go cheer him up. The idea was to play B&M together, but Marceline forgot to wear panties, which gave Finn a huge erection. Without thinking twice, Finn started fucking his friend, penetrating her hard until he finished inside her.
Marceline was angry because she might get pregnant. She was going to get revenge on Finn… maybe by practicing S&M next time.