We've got eyes and a biological drive towards reproduction on a primal level so I find it completely normal to notice when someone is attractive.
However there is a difference between noticing and leering. Single me found being leered at by someone out on date night with their partner just as uncomfortable as married me does. If they are both into it and whatnot they cool for them, I still find leering at people to be a bit classless.
When I'm out of a date, I might finds someone else attractive. My eyes see it and I internally think "that person is hot" and it takes all of about a split second for that to happen. I don't need to draw anyone else into that process if I'm out of a date and I find it to be derailing to my focus being on my date and the time I'm having with them especially now with poly since not all my dates are with one person. They have to share my attention all the time already, they shouldn't have to share it while I'm with them too.