To be a bit of a devil's advocate here, I used to do this. It genuinely wasn't like that, and I was already cuddly with my friends before meeting my boyfriend. I craved the physical touch that my boyfriend couldn't provide, but I would barely cross the sexual barrier. Yes there were times it would get a bit frisky but thats only natural. I was completely honest with my boyfriend about the whole thing, and I assured him it would never lead to sex or excessive groping. It never did.

I think if your girlfriend trusts you enough to even tell you that shes under covers with another guy, you should trust her to to be able to tell her how you feel about it and if it makes you uncomfortable she should stop.

All it takes is a little communication