Hypothetical situation: If your partner kissed someone else?

As the title says, how would you respond to your partner kissing someone else? There are several ways of kissing so i'll limit it to the following - A peck on the lips - Several seconds kiss - Full on make out session I'm just interested to see everyone's opinions... it came to my attention when a friend of my girlfriends said 'You were so close to kissing 'generic guys name' the other day' to her (I was at the table, not sure what the hell she was playing at bringing this topic up in front of us both) I wasn't paying much attention but I saw my girlfriend give a glare of death to her friend instantly shutting down the topic. So it made me wonder...if she did kiss someone else, how would I feel? Would I break up with her? I know for a fact I would be hurt...but to what extent?

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