Men staring at women while their girlfriends or wives are with them is very common because it's a man's instinct to look at an attractive women even if they are not alone. It's something that happens every day in relationships everywhere yet most women have no idea how to handle it when their boyfriend stare at other women with them in their presence. Your first instinct when your boyfriend looks at other women is likely to ask him what he's doing or ask him to stop. The truth is that most men do it without thinking. Men are incredibly visual and for them nothing is more appealing to the eye than a pretty girl. When the woman they are with sees them looking at someone else, what was suddenly something fairly meaningless to the man becomes an argument. You have to remember that your boyfriend isn't looking at other women because he's thinking or replacing you with her, he's may be simply appreciating how attractive she is.
There are things you can do when your boyfriend looks at other women that will actually draw him closer to you. Even though you may feel a very strong urge to make an argument about it or make an insult about the woman he's looking at, don't. It will make you look like you are jealous and untrusting and it makes him want to do it more. If you do you'll instantly appear insecure to him. You don't want this to happen. If you do happen to think she's unattractive or that she's dressed too provocatively, keep those thoughts to yourself. What you should be doing is either not reacting at all, or better yet, saying something positive about her. It's especially helpful if you notice her first and can point her out to him it will completely catch him off guard. It will also demonstrate that you don't feel threatened by other women. This suggests to your boyfriend that you are not only confident but that you trust him. Your boyfriend may be doing it just because it gets a reaction out of you and he likes the attention even if it's negative attention. If you take this new and better approach when your boyfriend looks at other women, you may be rewarded by more attention from him. He'll see that you aren't the jealous type which will make him appreciate you even more.
You're welcome and let me know if you need my help again. Good luck to you both!