Why do some people get more mad at the person their partner cheated on them with, instead of being more mad at their partner?

I just saw a video online where a woman caught her guy in bed with another woman. She started hitting the strange woman with a frying pan, but didn't try to hurt her boyfriend at all. I didn't understand why she made that choice.

(I'm not the violent type so wouldn't do either of these, but if I had to make a choice, I would rather hit him than her.)
Why do some people get more mad at the person their partner cheated on them with, instead of being more mad at their partner?
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