I have a girlfriend who is starting to bore me, the only reason I'm not going to end the relationship is because shes pretty good looking (good enough to make people I know and don't know come tell me I'm lucky). But I want to venture to new territories without getting caught.
Any advice?
You sir, are no gentleman.
Shape up and put an end to your relationship, clearly she deserves better than what you have to offer, don't deny her happiness.
EDIT: now I see your further reasoning, just talk to her about how your relation with her as it currently is doesn't 'work' for you anymore. At least let her know your not happy with the way things are going. Maybe you'll be able to work things out, maybe you won't, but at least both of you will (kind of) know what's going on.
Trying to get something on the side is just not fair, you're claiming more than you should while restricting your girlfriend, that's just selfish.