or no ?

I moved in with a female friend of mine, and soon after I started dating her best friend. My girlfriend recently admitted she wants to try a . After a long discussion about the potential drawbacks, I agreed to take part as long as the two women understood I would not have with anyone but my girlfriend. My girlfriend considers my roomate to be a worrisome threat to our relationship, so I was completely baffled when my girlfriend chose my roomate as the third party. Further, my girlfriend told me that my roomate finds me attractive, so I am worried that the experience will end badly. Any suggestions?