Take a good look at your relationship. Is this the only big problem you guys have had? Do you really care about her and want to have a future with her? Don't ruin something that has the potential for a lifetime thing over one goof up.
You aren't the only person she is going to be attracted to. Yes it was selfish and stupid and clearly lacked any thought. I'm sure she is sorry. She seems very sorry. You do not need to punish her for this though. Look into your heart to forgive her. You don't have any other reason not to trust her, right? She's honest with you about everything, right? I would kill to be in your position and have what you have right now.
You know what you are doing to her and this relationship. And it shows you care enough by coming on here looking for advice to help you out of your situation. Don't do something that you may regret later. Alot of relationships only get one chance and if everything else about it is working, get over this bump and move forward.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.