I would say that if EITHER the chronological age OR the physical body OR the inner person is a child, then adult should not sexualize.

The reason to be bothered even about fictional depictions is that it normalizes and facilitates abusive relationships. To take an extreme example, Jeffery Epstein. Everyone around him knew he "liked young girls" but due to overall cultural acceptance, everybody was able to pretend they couldn't notice the abuse going on and no one intervened. The comments in this thread are part of an effort to shift norms around sexualizing minors, to make it unacceptable and encourage intervention in such situations.

This topic has been litigated to death all over the internet. Surely there is FAQ or something that answers every single minute variation of question.