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it's not just because they're partners that hazuki is entitled to be in every crook and cranny of naoto's life, he's allowed to keep secrets and he's allowed to have other people than hazuki and the children as priority. Also naoto is not "too trustful" he's a kind and gentle person who got over what happened to him and chose to still see good in people, because he knows the reason he was raped wasn't because of something as fickle as being "too trusting" and if he ever came to be raped again that wouldn't be the reason either, what happened to him is the result of a society based on violent prejudices and of people's own internal malicious intent.