It is natural for a guy to look. It is almost a reflex for them to do so. BUT, it is completely uncool, especially for a guy in his 30's and beyond, to be so obvious that they get caught doing it. I can say most guys learn quickly how to be pretty sly about checking girls out. Even if they are single they realize a lot of girls wouldn't appreciate a guy gawking at them in a way that is obvious and they certainly know if they are in a relationship their woman won't appreciate catching them in the act.
I am not threatened by it personally. I used to spot hot girls and point them out to my last boyfriend. Not like they are going to go pick up on the girl. And, hey, if I think the girl is attractive how can I expect my man not to.
Yes it is totally normal for a guy to notice a "hot" girl who they happen to see, especially if she is dressed or positioned in a revealing way. On the other hand, that does not mean they need to give in to the temptation to look constantly! I have been happily married many years and still tend to notice other women a fair amount, but really try not to make a habit of it, which is more respectful of them and nicer to my wife. If your guy doesn't see the need to do this there may be an issue.
If I were out with a guy and he was obviously eyeing someone else, I would be offended. I suppose it is only naturally for guys to look at girls, but it can be done it a subtle way as not to embarrass everyone involved.