Marvel, at least in the early days, reportedly had behavior clauses in their contracts. Maybe that was just for Downy Jr., given his past?
Normally I don't agree with such things; judge a person's work on the quality of the work, not what kind of asshat they are.
But when you're playing a character who's meant to be a hero to little kids? You respect that and keep your nose clean. Nobody wants their child to find out that Flash is actually a douchebag.
As for the pronouns, I had no idea Millar went by "they" either. And honestly, with Millar I find myself not giving a ****; try being a decent human first, and then I'll try to remember the right pronouns. If you can't act respectfully, don't expect to be respected.