Just glancing at the Ninja thread bans, the thing is that what he's doing does fulfill at least one definition of sexism: Discrimination on the basis of sex. That much is undeniable.
However, the thing about using that definition to define sexism is that that definition also allows for justified sexism. Most people agree that public bathrooms should discriminate on the basis of sex--which fits that definition of sexism. But it's justified sexism.
Even anti-discrimination laws don't ban discrimination on the basis of protected classes and traits outright. They just establish a very high standard of justification. Your business needs to have a really good reason to discriminate on the basis of sex. But it can be justified and legal, if that burden is met.
What Era mods are doing is using one definition of sexism to indisputably label his actions sexist, then adding in the it's never justified nature of other definitions of sexism.
In short, the question isn't whether Ninja's actions qualify as sexism under any definition, it's whether or not his actions are justified.
Of course, you can't have that conversation on the message board built for discussing things.
