AITA for slapping my boyfriend?

My boyfriend admitted to ‘Not remembering’ whether he groped one of my friends on a night out. My friend was crying and begging me to believe her and after pressuring him to tell the truth he said he ‘couldn’t remember’ if he touched her up or not, and laughed in my face. I have never once hit him before but what he said absolutely disgusted and broke me. Up until this point he has been the sweetest boyfriend ever and to have that idea of him shattered in moments broke my heart it was almost like a reflex when I hit him. What upsets me even more is that when I have tried to speak to him about what happened he keeps acting like it was no big deal. My boyfriend and his Dad are treating me like I’m insane for being upset about what he did to my friend, and keep telling me it was just a drunk mistake on his part. My friends have told me I was in the right for hitting him and that he deserved it but I still feel really guilty. AITA?