Look he’s an adult and because of that I personally do not have an issue with it, but first and foremost that is YOUR house and those who are in it should respect YOUR rules.
If you are uncomfortable with it then you need to put a stop to it. They can sleep together at her place.
Ask him to leave the door open so you can watch.
He will think your weird and not want to do it in the house anymore.
Problem resolved.
He’s actually not an adult.
He may be 22, but he is not self supporting.