I know exactly how you feel belowthebelt.
My ex tried to pull the same on me (Only it didnt go to the level of her actually cuddling with some one else). She pretty much said that its okay for her to flirt with other guy, make herself seem available and lead them on because at the end of the day she will come back to me and i should "TRUST" her (The most rediculus thing i've ever heard because she would totaly flip if i did the same to her). I came down hard on her becuase i dont think this kind of behaviour should be tolerated in the relationship. So she decided to let me go and actually i felt happy about it. Because if your significant other behaves so disrespectfully and so shamelessly towards you then you definatelly deserve to be with someone else.
As a first step you SHOULD let your feeling be known and you SHOULD let her know that you are after a monogomous relationship and will not stand for anything that falls short of that. If she is unhappy about that then be prepared to move because this relationship will only end in tears. If she understands you and promises to change then proceed ahead with extreme caution.
How old are you guys? If she hasn't had a lot of experience in the relationships in the past then she might not be ready for the same level of commitment as you require and once again you are better off looking for someone else. Someone else is not that hard to find by the way (The hard part is letting go)
Hope above helps, good luck!!!
