Ah, this brings back memories of similar problems with my 3 DSs. Yeah I'd say he was embarrassed but also, maybe, a bit baffled or annoyed. He's what, 8? When my DSs were that age, we had to have a series of conversations about why boys can't hit girls. They experienced this, initially, as 'unfair' and 'stupid'. Some girls took advantage, they knew the teachers reacted much more seriously to a boy hitting a girl than the other way round, so they'd wind the boys up.
And, of course, between themselves, at that age, boys sort out all sorts of minor disputes and teasing with a punch. "Oh, he was being an idiot so I gave him a dead arm." And they don't snitch - as they would see it - about the punching, so girls who do are completely persona non grata! It's quite a moral tangle, and I tackled it from the angle of 'when you're a man, hitting women is completely out of order, any bloke who hits a woman is seen as a total arsehole, by both men and women, now why d'you think that is?' Followed by discussions of relative physical strength, notions of 'the weaker sex' etc etc.
Because they really couldn't see why they shouldn't be able to punch some strapping girl who was winding them up, who had - sometimes - punched them first. And the only response is, 'because society has rules'. Not laws - after all, it's just as much an offence in law for a woman to assault a man - but expectations of decent behaviour. They didn't get it at first, but I asked them, have you ever seen your dad hitting your mum? And then a long list of other people's dads. And they begin to get the message that it ought to be beneath them as males to use their physical strength to overpower or intimidate females. The discussion ranges over outliers like 'What if there was a woman who was, like, really strong, and she was punching some weak little guy?' And the answer is, well maybe he would be justified in defending himself, but when have you ever heard of that happening?
Your DS sounds like a nice kid, so I'm guessing he's just struggling with these moral issues. All I'd say is, don't assume he knows or understands why you're mortified, have the discussions. And can you get his dad in on the act?