On that one, I think Superman is an ideal more than just a character. In comic books, he's sort of the ultimate male ideal while Wonder Woman is the ultimate female ideal. Which is why people get so angry arguing which of them should win in a fight.
I think the idea of Supergirl (who isn't even the ultimate female comic book character) beating Superman really sets off a lot of core evolutionary or social attitudes. Maybe sexism but basically people get so preoccupied that she actually beat him that the very idea of a man hitting a woman takes a back seat.
I've been in martial arts. There are women who could beat me, one of my instructors, for example. But I'm not the symbol and ideal of the ultimate male.
I think Superman is and the idea of a woman beating him sets off that core reaction. Granted, anybody beating him sets off that reaction to a degree.
There were some things that happened in that fight that I winced when he punched her or slammed her around. But, when it comes to fantasy, we're long since in the era of Xena, La Femme Nikita, the Black Widow, and so on. We're in a time when it is often presented as women and men being pretty much equal in strength and that women can readily go toe to toe with a man of similar fighting skills. So Supergirl taking those punches and coming back with her own punches that are just as hard or harder somewhat mitigates the visceral reaction that people would have in real life to a man hitting a woman.