)
A fairly logical one. Putting a penis on a pony suddenly makes them naked. A humanoid character with jeans on and their fly open—with their penis out—wshouldn't be naked either.
The image that made me notice this was [spoiler]||>>2257239[/spoiler]|| (explicit). Adding @`nudity@` to this image (and any similar ones only containing ponies with a visible penis) makes no sense.