As SOS says, it's debateable. On the one hand, there is a long history of cluster outbreaks pointing to me/cfs as being an infectious disease (some individual cases suggest the same). But on the other hand, most of us are isolated cases not being any part of an infectious cluster or relationship and never giving the disease to anyone regardless of intimate proximity to others.
Many, including some of our best docs and researchers, believe those with an infectious onset, including cluster outbreaks, had caught a bug that was acting only as a trigger, rather than being the cause of their me/cfs. They clear the bug, but then are left with me/cfs. As you already know, any evidence of a single bug being the "cause" of me/cfs, remains undetermined, and is becoming quite doubtful to some on the front lines and in the labs.
In my 20 years with me/cfs, I've had several very intimate relationships with others without ever transmitting anything close to resembling this disease. I think this is true of the vast majority of us. What I see more often with people close to us getting this disease (or another neuroimmune disease such as RA) is only in those genetically related such as parents, siblings, aunts, uncles, cousins, etc. But those not genetically related, like my wife, will not contract the disease, regardless of how physically intimate we may have been. Again as far as I'm aware, this is true for the majority of us. I'm far more concerned about my kids getting me/cfs from me due to genetics, than I am to passing some infection to them, that may be the cause of this disease.
My first thought with your story, is to get some serious testing to rule out the many exclusionary diagnosis for me/cfs....and there are several. You, and especially your girlfriend, may just turn up something that's treatable. Meanwhile until you get this diagnosed, I think your concerns are understandable. I wouldn't want to give this disease to my worst enemy. But again, she may have something infectious, whether it sets of me/cfs or not.
As you know, there is no certain answer to your question.......but you may be able get to the best answer by weighing all the feedback from others. Also, as I said, I would go after further diagnostics. Real bummer to have to miss your family gathering for the holiday, I sure hope you find a way to make it happen