I think you haven't made much use of image-generating AIs. They're quite capable of reinterpreting images into different styles. A very common use case for me is to draw a sketch of something and then tell an image AI to turn it photorealistic. The "automated collage" approach you describe is simply not how they work, it's a common misconception. Image AIs very much can create imagery of things that weren't explicitly in their training set, they're not just regurgitating pasted-together snippets.
You're also assuming that there are no literal photographs of children's genitals in medical literature. Again, I haven't exactly gone looking, but I'm sure there are some out there. Doctors can't afford to be prudish.
And finally, you can get plenty pornographic without even specifically showing off genitals.