Is it bad that I fantasize about another man?

I posted this last night, but for some reason I can't see the responses, so I apologize for making a repeat post and deleted the old one.

Just as a disclaimer, I have been with my husband for almost 8 years, married for almost 2, and love him dearly. I would never even consider cheating on him or leaving him for someone else.

I have this fantasy about the man I babysit for. Again, as a disclaimer, I would never act on this, in this fantasy, it would only play out if he and I were both single.

I have been babysitting for him and the kids' mom for 6 years (she's his girlfriend/fiance, not really sure. They go back and forth).

I have this fantasy though of him perusing me (AGAIN, if we were both single). Like maybe I'm at the stove cooking for the kids and he walks up behind me, wraps his arms around me, and kisses me. Or I'm at his house and we have sex while the kids are at school because we are keeping us a secret from them, but then they get home and we have to play it off or something. Or even not just in a sexual way, but we are in a relationship and I live with him and the kids and I'm their step mom.

I'm not even into him though, but I still have these thoughts. I don't know why. Maybe because I'm so close with the kids, I'm like their second mom and even their extended family refer to me as such. So maybe since I treat the kids as my own and have this connection with them, I feel like, in some sort if alternate universe, I should be with their dad? I don't know.

Like I said, I have been working with them for 6 years, but this fantasy only started within the last year or so. Am I a terrible person? I feel so guilty about this. Is it normal, on some level, to fantasize about another man?

Let me say this one more time, I would NEVER act on this, I don't even have feelings for this man or want to be with him, it's just this fantasy I have in my head. So please keep that in mind before you try to call me a cheater, a home wrecker or anything.