Wait, so you are saying that Superman should never ever lose a fight? That is unrealistic story-telling. And I am pretty sure that Superman has lost plenty of fights in various comics, TV shows and movies. Superheroes lose fights all the time, usually to up the stakes, so that they can bounce back stronger and save the day in the end. It makes for more exciting drama when our hero loses the first round only to come back and win in the 10th round. It is a common trope in comics and movies.
When Superman has all his marbles, and is fighting for good, and goes all out, no, he should never lose a fight. There are very few beings capable of beating him legitimately and usually that's because he holds back.
I'm not saying that Superman can't feel defeat. Beating him physically isn't how it's done. Superman can be hurt by hurting others around him. Hurting innocent people. The greatest Superman story I have ever watched was called Apokolips Now, in the 1996 cartoon. Superman took a beating physically, but he wasn't defeated. It was when Darkseid did something so shocking and horrible that even after humanity overcame tremendous odds, Superman was actually defeated--even in victory.
Another example was a story called For the Man Who Has Everything. This is a brilliant story both in the comics and in the cartoons. In fact, it was so brilliant that the writers adapted the story for Supergirl, with Supergirl stepping in for Superman. It was a story where in order to come back, Kara (Clark) had to give up her greatest desires. It was a mental torture.
Superman's weaknesses are not physical--they are mental. Like General Zod said--"he cares."
Yes, using kryptonite can weaken him. That makes him vulnerable physically--though that didn't happen here.
Here, Superman was just a tool, dumbed down and weakened, just so he could go around telling everyone that Kara beat him.
No, your opinion is not fact. His getting whupped has happened many times, from Muhammad Ali to Kara and many more besides.
Interesting that you accuse someone else of not reading while you bring up Superman v. Muhammad Ali. You are aware that fight was done under red sun lighting, so Superman didn't have his power, right? Or do you think Muhammad Ali was able to beat a guy who can juggle planets with no problem?