From guys you're dating -

Not being good about getting back to you in a normal/polite amount of time isn't a necessarily a problem in of itself, but if you can tell you're the only one getting this treatment, it's a hecking huge problem. Usually it means he intends to keep you as a side chick. Also, even if you got past this for whatever reason and are the official girlfriend - this is a guy who obviously doesn't think very much of you.

In a related vein, if they're obsessed with the idea of you cheating, they think it's plausible and a sane way to behave because they want to cheat and either are already cheating or are seriously considering it.

From guys you're not dating -

The "nice guy" treatment. That whole mythology of loving a girl who never leaves a villainous guy is literally the story of a homewrecker, told from their delusional point of view. (Although I will concede that it is possible to try and get someone away from an abusive partner by improving their self-esteem without a romantic motive in mind)

Also, finally, I've got a developing story that I really can't boil down to a pithy statement yet. I've got this guy in my life (we're friendly, and I spend a lot time around him) who has a bizarre fantasy life where his wife is horribly mean to him, angry all the time, and doesn't "let" him do stuff. I've seen them in action. This is 85% him acting on the dynamic in his head. I'm pretty sure this is the opening gambit of a long con, but I'm calling this a red flag, and I have zero intention of finding out what the end game could possibly be...I guess he's trying to play on my sympathies? It's very weird.