For many men, the need to get these little voyeuristic peeks has given rise to a full-blown pantie fetish.  It's like an evolutionary adjustment to the fact that most girls still wear panties.  The object of lust will not be denied, even if it's covered by panties.  We'll just get turned on by the view of the panties themselves.  Some men even prefer the panties to a glimpse of nude nookie.  That would be a convenient fetish to have, I guess.  Personally, I like to see it uncovered, preferably shaved.