My girlfriend said this and I punched on her shoulder. Who is wrong here, what do you think?

We're in serious relationship and gonna marry next year. I met her today after a month and we were very happy and romantic. Were talking about her friend who had sex with her boyfriend and ready to marry another. (Sex before marriage is not common here).

girlfriend asked "won't she remember her boyfriend' when she have sex with another guy after marriage"

I said "I donno. I've never thought about it with another woman"

She said "I've thought about it"
I really got sad and shocked. I told her "tell me one more time if you dare" andShe was like "I've thought about doing everything with other guy".. Tears came out of my eyes and I punched her shoulder really hard. She cried and later said she thought it was funny. What do you think?
My girlfriend said this and I punched on her shoulder. Who is wrong here, what do you think?
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