I was fantasizing like crazy about my girlfriend in the beginning of my reboot and I am very satisfied with my recovery.
You want to go change yourself to being a man, who likes real women and has a healthy sexuality. Meaning you like women that are real and you actually talk to and woo and make your girlfriend/partner/sex buddy/wife. Something real and not just one-sided fantasy. So imo, all that is related to walking that path is OK, including having fantasies about them. Even sexual ones.
I should point out, that I do not believe the "monk phase" of a few months of abstinence and absolutely no sexual thoughts is needed in a reboot, if your main goal is no porn for the rest of your life and you actually manage to start living that way. You can rewire from day one of your reboot. And I assume you want to rewire with the person you are fantasizing about.