How do I answer this they are 17 years old?

my daughter has 2 friends over we live in Florida it is currently 79 degrees. my daughter and her taller of the 2 friends has a one piece bathing suit on and the other friend has jeans and a t-shirt on. my daughter came to me and asked if it was ok for the girl in jeans to be in her underwear i said no.

How do I answer this they are 17 years old?
Post Opinion