I found this thread by searching "fantasy other men" and I had to comment because of the parallels between me and your gf lol

I'm also 19F with a bf that is my first ever and I've been starting to have frequent fantasies about other guys. We've been together for almost two years and I think for me it's curiosity how sex would be with someone else. Also it's probably natural for things to not to be as exciting when you're with one person for a long time. My boyfriend doesn't like being in an active role during sex while I would really love for him to be. On top of that he's reluctant to talk about it, says it's impossible to try because he just doesn't have the personality for that. I'm not sure this is very helpful to your situation, I just wanted to share my experience somewhere lol

Fantasies are fantasies tho, I would never actually cheat on my boyfriend. I'm sure your girlfriend thinks the same way. Maybe you can voice your fears to her, I hope you talk it through! :)