There is little more insulting than a wandering eye.
I feel about this like I do about the necessity of porn sites and a boyfriend subscribing to any magazine that arrives in a black plastic bag.
I don't know that I don't agree with it as much as I don't understand it.
On the other hand, I know that there are certain movies I will see just because Vince Vaughn is the star, or go to a concert where the musicians might be better looking than they are talented. Its natural for people to look and be attracted to people other than their partners, but there is looking and there is eyes-bugged-out, chin-to-the floor, let-me-help-you-put-your-tongue-back-in-your-mouth looking.
From your letter, obviously this is bothering you, but before you bring it up, think about if the real problem is that he's looking at other women or if its that he isn't paying enough attention to you. The two often go hand in hand if the wandering eye wants some other action.
If he is attentive to you, let it go.
Return the Behavior
Better yet, return the behavior and see how he likes it. When you see a handsome man, rather than blush and turn away, smile at him. Let your boyfriend know that you can play the game, too, and maybe he'll shape up.
From your letter, I'm figuring he's in his early 40's at least, which makes him an old dog who can't learn new tricks...but if you really have a solid relationship, maybe he can be your puppy once again.