I feel like a terrible boyfriend.

I am a man with a girlfriend, she means the world to me. I care about her so much. Our sex life I good. But whenever I'm alone I alway fantasize about being with a man. I'm sorry if this is wrong sub or of I'm breaking any rules. I just feel like a bad person because I know she loves me, and I love her, but I can't stop thinking about wanting to be with another man. I know if she found out it would crush her. I just feel like I needed to get this off my chest. I don't know if any of you have gone through anything similar, or if you know anyone who has. I just need to talk to somebody.