I need advice. (Warning: I talk about sex)

Has anyone ever thought of someone else during sex?

Okay so here is the details, I thought of another guy during sex but I didn’t think of being with this guy solo. I thought about it with my boyfriend (such as a threesome). I don’t want to screw this guy, it was just a fantasy.

Here is the problem, I’ve been with my boyfriend for 5 years, I tell him everything. It was eating at my heart for a couple of days so I had to tell him that I thought of another guy.

He isn’t mad but I can tell that I hurt him. I don’t know what to do. I love him so much which is why I told him. I don’t know why I thought of this other guy! I need help! I really feel that I have ruined my relationship and have been really stressed all day about it.

He keeps telling me I’m fine and it’s okay but I just feel that it’s not. I don’t want to hurt him and I don’t want the relationship to cease.