There's a fine line between "fun absurd" and "stupid absurd". The notion that a person could build a high-tech suit of armor (Iron Man) is fanciful, but it is only x degrees from reality. The idea that someone could have mutant powers is x+5 degrees from reality. The idea that a talking raccoon would join a band of superheroes is x+50 degrees from reality.

At a certain point, they've jumped the shark and reminded you just how stupid and ridiculous it all is, and you say, "Ok, fuck it. This is just stupid and a waste of my time." That point is different for different people - my father would say, "I'm not going to watch something ridiculous like that" already at Iron Man. He'll watch the Die Hard movies, even though they're far-fetched based on the laws of physics, but he's not going to watch superheroes or elves or pixies, or aliens blowing up the White House.

I draw my line at Avengers and X-Men.