As a man, it's not always about being a perv. Sometimes a girl is so good looking that it's impossible to not want to look. It's like looking at beautiful scenery in nature.
Of course, some are pervs and I don't speak for all of us. Just for me
Bro you don't suddenly start being able to see when you enter the sea.
I have -6 vision in both eyes, if I took my glasses off at the beach I would bust my feet up not knowing what I was standing on.
(For reference at -6 dioptres perfect vision is at max 10 cm from my face)