Right now it is not a reality, but an idea. Which means you (hopefully) have time to think about it, talk it over, and ask any questions you might have. Keep the conversations with our gf open and honest and let her know your thoughts and fears. It doesn't seem as if you need to come to a conclusion immediately so give yourself time and relax. I've had one true poly relationship since being married and all of these thoughts filled my head as well. Just as they fill my head again now when considering seeking a new relationship in the future (first didn't work out).
I do wonder what it is about men specifically that threatens you. As a bisexual woman married to a man, I can see why he isn't up for opening our relationship to another man. But a woman does not threaten him as he could never provide for the same physical and emotional needs as a woman. I'd like to understand your reasons should you figure those out. And maybe some of the men here can help calm those fears.