We let our DDs sleep with their boyfriends at our home at the point when it became clear that these were serious stable relationships - the actual age at which this happened differed for each DD, depending on their maturity and the length of time that they had been in their relationships.
Can you speak to your own offspring (i.e. GS's parent) about what the policy is from their point of view and why; and ask what they think you should do? - with the proviso that, if you are not happy about it, the "your house your rules" rule should apply.
What age did you reach your current height?
What was the name of the corner shop you went to as a child?
I would go with what others have said, it's your house. Separate rooms

