My girlfriend wont do anything with me and has a lot with other people.

To make a long story short, i have never had a threesome or swinging situation and really want to experience it. My girlfriend cant even count how many threesomes and orgies she has been a part of and when we first dated she straight out said we can totally have a threesome. But now she says absolutely not and gets pissed anytime its even brought up. I have even offered to give her one and she flips out about that too. I feel like she was telling me what she thought i wanted to hear and im a little upset about it. Im not a dick and dont want to force her to do anything that she doesnt want to....but wtf?!?!. I obviously think about it constantly and dont know what to do. I dont want to cheat on her because i would like her to be a part of it. I dont know where to go from here, i need advice/help...Should i just give up on ever experiencing these things even though i am not comfortable with that at notion at all? Thought?