I had an ex who did this. If I had been a smarter man, she would have been an ex immediately after it happened, but I continued to be with her for years afterward.

We were arguing, I don't remember what it was about, and she just let loose and slapped me in the face. I had been mad before, but the physical pain was enough for me to see red. I used to have anger issues as a kid, and this would be the exact moment where I hulked out and my parents walked in and found me standing over the crying heap of one of my brothers. It's been a long time since I ever got that mad, and I managed to collect myself and communicate my feelings instead. I think she could see it in my eyes, though, because she suddenly looked very uncertain of herself, and I said:

"Don't you ever do that to me again. Ever."

She defended herself by saying "You're a guy, it doesn't hurt you."

I proceeded to explain that maybe it didn't hurt that bad, but no one deserves to be hit. It's disrespectful, it's demeaning, it shows that you don't respect me as a human being. You don't hit someone to get your way. That's something you're supposed to have figured out in elementary school.

It wasn't the last time, though I never got slapped in the face again. Occasionally she would get mad and try to punch me in the arm or something. It wasn't a playful sort of arm punch, either. It was an "I'm frustrated and pissed and I'm hitting you to take it out on you." sort of punch. Most of the time I'd manage to catch her hands, pin them down, and tell her again exactly why it's disrespectful, and why it was NOT okay if she wanted to be in a relationship with me. Would it be okay if I hit her? No. Then why is it okay the other way around?

She'd go back to the same excuse. "Well it doesn't hurt you when I do it, I'm not strong."

It eventually stopped, and we were together a few years after that before I eventually broke it off because I hadn't trusted her for a very long time, that lack of trust killed my sense of intimacy, and I never really had a 'good opportunity' to leave, since we both shared an extensive friend group and we were in college and going through a breakup would have been miserable.

I'm not a smart man, I guess. I try to make things work for a long time before I'll finally admit they're broken. Assault is never okay, even if you're a big tough man and it doesn't hurt. If someone hits you in a fit of anger, it shows you how they really feel about you. That you're something in their life for their convenience and if you're not meeting their expectations they're going to try and force you to.