Here's the thing...
she's just looking at the other guys, but she's with
you! We all look at other people, even when we're in a monogamous relationship. I'd think that even you do this, based upon your avatar
If you're really worried about being able to keep her, you need to ask yourself if her looking at others is the only catalyst. If it is, then don't worry! If it's not, then talk to her about it. I don't think it can be said enough: communication is key.