Why do men do this? It is so annoying!!!!!!!?

Why do men look (and don't hide the fact they are looking) at other women when they are with their girlfriends? My b/f did this to me, he basically drooled over this other women who was passing by us. I let him know I saw what he did (jokingly) but I started thinking about it and getting more upset. It makes me feel like I'm not good enough. Why would he need to look at other women? Next time he does it I may get cross with him....>:[