Recopying this reply b/c of issues with amazon link
To make your wife happy and you happy...there's actually a solution here! Use a 3-way switch on the light and a humidity sensor on the fan, wired into the light switch in series so that either the light or fan sensor is energized at any point in time. The fan will run only when the light is off AND only when humidity is high enough.
Basically, the three-way switch would get wired to the light fixture and fan sensor on the two traveler poles. Then the fan sensor would be energized only when the light fixture is off (and vice versa). That way, it'll dry the room out after the sauna experience is over and not during shower time. That should get you at least a few extra months before you have a mold problem again! I'm not 100% sure this is a code-compliant use of a 3-way switch, however, so someone with more knowledge should weigh in on this one...
https://www.amazon.com/dp/B00H3QQD64/
Also, there are mold-resistant paint additives that might be worth doing as well. I bet you'll solve the problem with the combo of both of those solutions.