I think it's a combination of two factors. One is that men are visually stimulated, and the second is that we are socially conditioned from a young age not to practice self-control and just do what we feel like. This is seen as very "masculine". If a woman does the same thing though, she is called a "bitch". It's terribly unfair, but that's our society for you!
Simple. Ratings.
Because they assume that you think like a man. A woman does not think like a man does. Evolution has programmed us with different strategies when it comes to ensuring the survival of our genes. A woman desires a man who will stick around to raise the children, and who can protect the family from predators. A man desires women who are young and healthy, and can bear healthy offspring. Women get upset when men ogle other women because of the fear it induces. They fear that the man will break his commitment to her and run off with the other woman, whereas the man doesn't see any logical reason why he can't lay BOTH women. Just because he's f***ing one doesn't mean he can't also be f***ing the other. To him, his thinking seems perfectly logical. His genes benefit most by having sex with as many women as possible, whereas the woman's genes benefit from having a man stick around to ensure her offspring survive and thrive.
More social conditioning here, IMHO.
It might be worth asking why you would get upset, and why you think getting upset would be silly. Is it that you know he will still be faithful to you even though he can't help but notice when a hot chick walks past? Is it that you will inevitably find yourself comparing yourself to those other women and feeling inadequate as a result? What, specifically, do you fear? (I don't know what the answer is, just some suggestions for things to think about.)
And it frustrates me too that so many of us men don't practice better social etiquette. I guess the best I can hope to do is to try to practice it for myself. I can't change what anyone else does.