How would you react if your partner tells you this?

If your wife/hus or partner in serious relationship (you're gonna marry) tells,

"If I do it with any other partner again in future, I'll remember your face when doing"

I asked "You thought about doing it with other people? Tell me that one more time"

Partner: "Yes I've thought doing everything with other guy"

I got sad, angry and shocked and i punched her shoulder.

How would be your reaction if your partner told you this?
How would you react if your partner tells you this?
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