What is that thing line between staring and seeing when he is taken? What does it mean?

They say that even men who are very devoted to their girlfriends may sometimes secretly stare at other women knowingly or unknowingly.

Some men make it so obvious.
He stares a lot when his girlfriend isn't around. Even one time, he looked shocked with his eyes wide open and mouth open. And scanned my body by checking me out.

They say that there is a thin line between seeing and staring.

Just wanting to know more about men like him.
No I'm not a homewrecker.
What is that thing line between staring and seeing when he is taken? What does it mean?
Post Opinion