Pretty much all my life I've seen in movies and tv shows where if a woman is upset with a man, she slaps him. Guy upsets her, insults her, makes her mad, she slaps him across the face. I've also heard all my life that "boys don't hit girls" but how many people have not bothered to teach the reverse to their girls? Consequently, a lot of women seem to have grown up with the idea that somehow a woman slapping a man isn't as big a deal as a man slapping a woman.
Men very often don't report it if they have a spouse who hits them. It's embarrassing to admit. Someone I know well was in a relationship like that, and he would pass off his bruises, scratches, and black eyes by saying that they happened doing sports. We all believed him. The last time she did it, he'd finally had enough and he called the police. They didn't actually arrest her but she was charged. Previously to that, she apparently felt like a woman hitting a man was not a big deal, and only women could be victims of DV, so I think it was a huge wakeup call for her.
I'm not saying the OP did anything like that, maybe this truly is the very first time they argued and she had never slapped him before, but I have to agree with leslie82, if this was the first time they'd EVER argued and she got mad enough to hit him, then that to me is worrying. Seems like once you've done something like that, it becomes easier for there to be a next time.
I say this to everyone on Facebook or when there is a discussion about men hitting women. Or I see the memes "real men don't hit women." I say "NO ONE should hit anyone if it's not self defense." It is wrong for women to abuse men. It's wrong for anyone to abuse their loved ones. Domestic violence is gender neutral.
If I had the money and ability I would open a shelter for men who are victims of domestic violence or find a way to set it up that men and women could be housed in a shelter. Different dorms or something separated so neither can be triggered. Because there is nowhere safe for men to go sadly. Men are not allowed at really any dv shelters.
Women hitting men is not a big deal. Ever see the scenario they did on "What Would You Do?" A woman slaps her boyfriend in the park and berates him and shoves him. Most people said "He probably had it coming." Or laughed. But a man was shoving a woman and it was "oh hell no."
Our society seems to think men should always be strong and women are weak.