Do you think she could possibly be feeling "judged" by the "poly community"?
I've noticed a certain, I don't know a polite way to say this - disapproval on this forum and elsewhere toward people who say that they have had casual sex, do not regret it, and would do it again if they had the chance. It seems as though the correct answer to "how do you feel about casual sex?" is "I tried it and it felt so wrong that I will only have sex within the context of a "loving" relationship from now on."
Even though there are probably just as many if not more folks on this forum who enjoy casual sex and are not afraid to say so, I think that the term "polyamory" CAN connotate casual sex as something people do because they don't know any better.
For the record, I am one of the individuals who DID used to enjoy "casual sex", "sport-fucking", "friends with benefits", etc. and while there are certain incidents in which I would have been better off not taking part, overall I do NOT regret it, and would for the most part do it all over again. So, I say to your girlfriend, "Go girl", as long as you are up-front about it with each other, and practice safe-sex.