Consider the following three activities, all done for a sexual end and without the consent of the other parties:
- Wearing special "x-ray" goggles that show one what other people look like under their clothes
- Wearing special computerized goggles that quite accurately extrapolate from the visible features of other people and from visual data about how their clothes lie on them, using a large database of body types, and show what other people very likely look like under their clothes
- Walking around and using the visible features of other people and visual data about how their clothes lie on them to imagine what other people look like under their clothes.
The argument doesn't apply to every case of lustful fantasy—I think there are other arguments, like this one—but I think it captures some of why many cases of sexual fantasies are wrong and creepy, indeed are a kind of non-consensual sexual relation.