I think a parent must do what they think best and what they feel comfortable with.
In our case, our son's girlfriend stayed overnight in his bedroom (in fact she more or less lived with us for two years). They were both quite young - around 17/18 - but sensible. I was very fond of her and, although they broke up years ago and she is married and my son has a partner and children, they are still friends.
We had a stricter attitude with our daughter (there are 7 years between them) which didn't work out at all well as she became extremely rebellious. We caused her, and ourselves also, a lot of unhappiness and we resolved not to make the same mistakes again.
COUNTRY, CITY, AREA, PLACE -Game 21