Why would a man verbally abuse his partner when he is confronted?

I caught my boyfriend on pretty much all hookup websites, active, the second after he texted me the sweetest message and after the best night we ever had together. So I confronted him saying I saw him active, but it was okay if he is honest with me. I wanted him to just let me know that he is looking for something, still. I was very calm and I gave him a chance to explain. For two straight days he would either avoid it, insult me, calling me a “whore” and “cunt”, “get a life haha” as well as cussing me out multiple times saying I like to make-out with guys (I made out with someone months ago and he found out, but him and I weren’t even dating then.)... He brings out these unrelated things and calls me a whore, etc. when he has slept with 40 girls and I only slept with 3 guys in my entire life. It makes no sense, but his abusive words and smirks that come along with it makes me wonder: why? Why would a man do this when he is confronted by a girlfriend? Why can’t they just say okay I screwed up, Im sorry?