People have different attitudes toward sex, sexuality, and committed relationships, which is totally fine...but if yours and hers don't quite jive together, then it can become a problem.
I think if you feel this strongly about it, then you should have a serious talk with your girlfriend about it and tell her exactly what your feelings are on the matter. If you don't, and decide to either just "go along with it" to avoid conflict or allow her to explore on her own without her knowing how it affects you, and you'll end up resentful. If she loves you, then she would most certainly value your feelings enough to come to a peaceful agreement with you. Let her know what exactly it is that bothers you when it comes to "contact improv" (which I've never heard of, so this is a new one for my vocabulary!) - you're clearly not comfortable with the idea of yourself participating...and you're uncomfortable with her doing so as well, which I completely understand - something like that can easily feel like infidelity, and it can make you wonder, "Well, am I just not enough??"
Chances are that this has less to do with her being unsatisfied with you and perhaps more to do with her approach to sexuality in general.
I think it would also be a good idea to communicate your fears in this as well - that you aren't "enough" for her, and that you're afraid she'll resent you because of your complete lack of interest in participating in this. Basically, I think a good way to deal with this is to get everything out into the open - keeping secrets when it comes to things like this can really hurt you both, and your relationship. At the same time, listen to her side of the story as well, just so you understand where she's coming from - what is it about this that interests her?