Guess it depends on how you look. If a pretty girl or handsome
dude walks past and you cast a glance at him/her and think to
yourself, "Mmmm... Lovely!", that's natural.
But if you're with a date and a pretty girl / handsome dude walks
past and you glare at them transfixed then you turn your head to
look at them as they pass by, disregarding the feelings of your
date, then that's really not nice. 
Like an American guy once said to me, a gentleman should never give
his girlfriend cause to be jealous and insecure. 
