I think what you're feeling is natural. Maybe you're missing affection, or maybe the guys remind you of your guy. Whatever the case, try not to feel guilty. You're just being human. If you're ok with the distance, then respect the commitment you've made to your guy. When feelings for others come up, remind yourself of the sweetness of the good thing you have. If you're not ok with the distance, then you and your guy need to have a talk about what you're feeling.
My fiance and I live about 3,000 miles apart for about 3 years or so... I feel your pain. :-|