Most Helpful Guy
You may not feel it that way but some girls just don't care if they are seen in their undies. At 16 my wife wasn't a 'slut' (she never was!) but coming home from HS, she'd undress to bra and panties, to change in a hurry in presence of her mom , dad and best male friend. "nothing more to be seen than on the beach, if you can't stand it, don't look" she would reply if her dad said something about it.
One of my female friends did the same (and more) at college. They just didn't care. Others wouldn't show an inch of cleavage or sear shorts.