Why is it that when a guy cheats on his girlfriend, the girlfriend goes after the girl he cheated on her with?

And vise versa. I’ve always thought that was so stupid. A guy meets a girl, they start dating, he just “forgets” to tell her he’s already in a committed relationship with someone else. The original girl friend finds out, and goes and keys the other girls car or something. That has NEVER made any sense to me. It’s the guys fault! Or it’s the woman’s fault if she cheated on her boyfriend, but her boyfriend is gonna go beat the other guy up!