This is incredibly flawed. It can never be behaviour observed in a vacuum. A behaviour's motivation in context is the only thing to observe and - potentially - classify as sexism.
If your perspective is one where you care about "culprits," and whether they are good or bad... motivations matter.
If you're characterising the system, not judging culprits.. you don't care about motivations, historical reasons or such. You just care about how the system behaves.
A sexist office is one where people are treated differently based on sex. That could be because of office rules. It could be because of cultural or religious reasons. It could be because of #metoo. Etc. Say you walk into that office, experience or observe sexism and walk out. You don't know why it's sexist, but you do know that it's a sexist office.
I get that you don't want everyone inside that office painted as sexists. Maybe they're not. That doesn't mean that it's not a sexist workplace. This isn't a criminal trial.
Criminal trials are the way they are because that's the best way we know to find out truth. If you don't care about truth, then just say so directly. No need to mention whether something's a criminal trial or not.