Thanks for reaching out to me here at Just Answer
Let's see if I can answer your question....
OK.... You did the right thing to reach out for guidance on this issue. Of course there is a difference between fantasies and real life and you seem to realize that quite well. It is of course wrong for this to happen in real life. However, fantasies are different as you point out.
Many couples role play and even "act out" their fantasies and many sex therapists promote this to help the relationship - they would say something similar for masturbation.
Having said that, as long as you keep it to your fantasies most would agree that no harm is being done.
I would only caution you to not become obsessed with any fantasy or with masturbation (if possible). I must tell you that in my experience, it is best to find a good and trustworthy mate to share love with and enjoy each others companionship.
I hope I've answered your question and I wish you well. Please let me know how it goes.