Superb post by Karl. I couldn't agree more.
I'm often "evangelising" on here about this.
We are terrible in this country about not keeping the "whole" of our properties at a constant level of heating and humidity.
As an example, my own house is rather large, and mostly open-plan.
Largely because of underfloor heating, the whole building is kept at a constant temperature, even rooms that are seldom or never used.
There is, of course, a high level of insulation.
Recently, the fan in my en-suite broke down. I showered today without it. Nothing steamed up. Although, I shall replace it to reduce amount of water vapour held in the air.
Open the window by all means, but do consider fitting proper extraction, if only to ensure adequate air-changes.