I feel guilty. In fact, this is one of the most common fantasies that people report. It is not something you need to feel guilty about. In fact, I bet your husband has occasional fantasies about other women. None of this means that there is anything wrong with your relationship or your sex life.
Studies show that people have sexual fantasies several times a day, or even several times an hour.
The point is that the ability to have creative, imaginative fantasies is one of the things that makes us entirely human. This is not only highly unrealistic, but leads to much unnecessary misery. I have even seen individuals leave a relationship because they were worried that a fantasy meant that they were doomed to be unfaithful. Just because we fantasize about something does not mean that we need to, or even want to, act upon it. I am much more concerned about people who insist that they never have even a passing thought about a person other than their partner.