Humans DO look at and notice others - not just men but women too. How they do it and where the person's brain goes from there is what determines whether it is a problematic behavior. If he checks someone out in an obvious way while he's with you and he knows it bothers you then it may be rude behavior or even degrading behavior BUT if he merely notices the occasional attractive person and then resumes interacting with you it is truly no biggie. Also, if he notices someone attractive and then begins thinking of how he can get in their pants without repercussion it is a problem, otherwise it isn't. If he notices others BUT has clearly picked you then it is a compliment to you. I'm sure you are beautiful to him but the reality is that we all find people other than our spouse "beautiful" too - it is our history together, our love of other things about the person etc... that makes them irreplaceable in our lives no matter how gorgeous another may be.
If he is "checking" someone out in an obvious and rude way in front of you then call him on the rudeness but not the checking out part. If you get too freaked out about him looking then it makes you seem insecure and a little immature whereas insisting on being treated with respect is mature and confident.
In the meantime I think you need to pursue some things that make you a bit more confident in yourself - do you do something for your own creative expression? Music, art, scrapbooking, gourmet cooking, dance? Find something that makes you feel great and do it.
