I can’t imagine not being polite if the other pilot prefers to wear a mask, but according to the latest union email, not even the captain can mandate a mask when the door is closed. If there is a problem, the person insisting on the mask is pulled from the trip. The mask drives me nuts and steam up my glasses enough to be distracting. It’s not a safety issue in a simulator, but it is in the plane. If someone were to ask that I wear one with the door closed I’d comply providing that they fly every leg. I don’t really know that any of this is an issue on the line though. I haven’t been to work in a while, but from what I understand, everyone takes them off as soon as the door closes. It’s almost like the race to get the shoulder straps off when the gear comes up.