Most Helpful Guy
it's instinctual, can't remember where sorry, but a while back I was reading about some research done into this and the conclusion was that it's impossible for men to never do this. The good news is that it means absolutely nothing, the men in the test couldn't even remember the women they checked out.
The only problem with it all was how women tend to not be able to relate and often make a big deal out of this when it's really nothing at all.
So it has nothing to do with being "pervy" and everything to do with being a healthy adult male.