It is hurtful to cheat.
Tiffany, who has a crush on Peter, asks him to come over her house while his girlfriend Sarah is out of town.
Peter wants to spend time with Tiffany.
Peter and Tiffany watch a move while they're at her house.
Peter starts making out with Tiffany while he's at her house.
Peter enjoys the move and has a fun, platonic time with Tiffany.
Peter tells Sarah out of guilt and she is devastated.