Is it bad to have romantic/sexual thoughts about someone else while in a relationship?

I love being together with my boyfriend and all, but I have this other guy friend who is also really cute. He liked me at some point, which my boyfriend never knew about. One day he told me he had a sexy dream about me which kinda turned me on. Almost every night before bed I think about my guy friend and what it’s like to kiss him. But I don't know if this is considered cheating, We’re not actually smooching in real life.

Is it bad to have romantic/sexual thoughts about someone else while in a relationship?
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