Without being there we can't say if he's a sleeze or doing nothing wrong.
We all notice people around us. Sometimes our gaze stops a second longer, or we look at someone twice, for various reasons (they remind us of someone / we like their coat / their hair colour is exactly what you've been looking for / they're drop dead gorgeous).
Any amount of obvious looking / leering / sleezing is always wrong. It makes the recipient uncomfortable and is disrespectful to them as well as to the person you're with. I can't bear those men who openly leer at women. It's vile, intended to let the person know they're looking (either intentionally to make them uncomfortable or on the offchance they might throw themselevs at the middle aged lech who's staring at them).
If my OH noticed a woman was attractive, I wouldn't mind that. I would also notice. Like I notice attractive men sometimes. It's a split second glance that noone else would notice, and you've forgotten yourself immediately afterwards.
If he leered at them, if he didn't care who noticed, if he was always noticing good looking women, if he looked every woman he met up and down, if he didn't care how it made me feel or the other women feel, then I couldn't be with him.