Dear Russ, Your situation sounds all too familiar to me. I met my current boyfriend a few months ago on a two week educational trip. We feel for each other instantly but one thing stood in the way- I had a boyfriend at home. He was adamant that nothing could happen unless he knew that I would break up with my boyfriend when I got home- I said I would and I did but nevertheless my ex and I have remained good friend. MY current boyfriend lives three hours away without traffic and so it is hard for him to know that I am still close with my ex who I have not hooked up with since - my point? It always hard to know that the person you care about is far away and around other guys/girls especially when its other guys or girls that they have been close to or even intimate with- unfortunately for you Denise has not remained 100% faithful and that leaves you with a dilemma.
If you ask me, its a trust issue. An issue because she said she wouldn't sleep with Rob and she did- she has done so, not once but a number of times. I understand that you are very much in love with Denise but can you trust her? If she has done this once what's to say that she will not do it again? If she is interested in making a concerted effort to be together and stay faithful I would say that guidelines and rules must be set- if she does not stick to them then you will have to make a decision- sometimes you have to make changes or take a stance even when you know you will be hurt in the process. It is all about learning about yourself and relationship as you grow up. The fact that you lost your virginity to this girl means that you will forever share a very intimate bond and that is going to make your decisions biased and very hard.
Just remember that the most important person is you- you must remain true to yourself and what you know is right for you...try to be logical rather than idealistic. Unfortunately sometimes these things happen and they are not easy. Long distance relationship in general are difficult - especially when one of you has already broken a trust.
Good luck and remember that at the end of the day you answer to you and nobody else- consider yourself when you are making your decisions.