Thinking about other men while in a relationship help

So I love my boyfriend he’s amazing but he is the only guy I have ever had sex with. I’ve always had a fantasy of having a one night stand and being able to have awesome sex with a stranger and then never see them again. When ever I masterbate I can only cum if I think about having a one night stand with someone. It makes me feel horrible and guilty like I want to cheat on my boyfriend when I would never do such a thing. Is this bad ?