The problem lies not in the fact that he notices a pretty face, but in the fact that he "expresses admiration and excitement for another".
Naturally, we can't stop ourselves from noticing an attractive person on the street, but it is entirely inappropriate and unnecessary to "express" this - especially to or around a significant other. If your boyfriend is flirting with another girl, it is an insult to you because it is a form of disrespect. The same goes for girls who flirt with other guys to make their boyfriend jealous. It comes across as a total lack of respect for your significant other.
There is a big difference between commenting on the looks or sex appeal of a famous person or movie star - because they are unattainable and non-threatening. But if the person is sitting next to you in a bar, or even a friend, then they ARE attainable. The possibility IS there, if they wanted to make something happen. And that is uncomfortable for anyone to feel, because in the end, if they like you so much better, why would they waste time commenting on someone else that they find less attractive/interesting than you? And if they think the other person has something that you don't, then whats stopping them from going and getting it?
In the end, people can (and will) look, but there is absolutely no reason they have to open their mouths and comment, or flirt.