Generated AI CP should be illegalized even if its creation did not technically harm anyone. The reason is, presumably it looks too close to real CP, so close that it: 1) normalizes consumption of CP, 2) grows a market for CP, and 3) Real CP could get off the hook by claiming it is AI.
While there are similar reasons to be against clearly not real CP (e.g. hentai), this type at least does not have problem #3. For example, there doesnt need to be an investigation into whether a picture is real or not.