Well, with me, personally, I don't consider it cheating, as long as the girl is up to my standards and has no issues involving me in the "activities" that may transpire. But if you are not into that and you want simply her to yourself, and you DO consider that cheating, just talk to her about it. Approach it calmly, and sanely, and simply explain to her how you feel about it. If you and her are meant to be together, she will understand what you are saying, and work on it, or you two will work something out between you. Because at this point, if she is doing that, she doesn't think you consider it cheating, if she knows that you know, so she will keep doing it. So just talk to her about it.
Also, Archange1. I feel as if that response was aimed more at me, but whatever.