I cheated on my boyfriend with a girl and he doesn't consider it cheating??

Michelle
It actually happened today and she's an old best friend & actually told me after we hooked up that she wanted me to dump my boyfriend and I'm not going to because this was a one time thing but I told him & he told me he doesn't really think it's cheating. I feel super bad but I'm also like dude??? Shouldn't you be mad?