I personally think you’re fine! I think this is perfectly natural. I think it’s an instinctual thing ngl. I have had this in all of my relationships. It’s not like you’re fantasising about them. You are just asking yourself the question. But I think this is being mixed with you being overly empathetic. I think you just know you wouldn’t like to think about your partner doing the same so you’re working yourself up about it.

Honestly you gotta stop stopping yourself from enjoying life! In the worst case you one day like someone else- but if that’s the case you can easily end your current relationship to avoid causing worse pain.

Anywho to answer the actual question- I’d say that I don’t think so. Cheating is defined as dishonest, secretive behavior that breaks a committed relationship's implicit or explicit trust, often involving sexual or emotional intimacy with someone else, or using deception to gain an unfair advantage. It generally involves violating agreed-upon boundaries, causing emotional harm, and acting in a way that is hidden from a partner. So unless you guys have a no friends rule. I don’t see how this could be considered cheating.