Why do I have condensation in my bathroom after taking a shower despite opening a window in the room?

The lack of ventilation has a lot to do with it.

Condensation forms when moisture is attracted to a colder surface,Cold cannot hold moisture as well as hot.

A window opened isn't enough airflow to compensate for the amount of moisture in the air.

A ceiling mounted exhaust fan would lessen the moisture in the air.

KBC

Yep, look up a dew point table. Moisture forms on a cold surface depending on the surface tempurature and the temperature and humidity of the air.

That's why I have a delay-on-break timer on the fan. It runs the fan for about 30 minutes after the switch is turned off.

Panasonic apparently makes some of the quiteter fans. I use one with a heat lamp, fan, and regular bulb although not Panasonic.