The thing that makes pedophilia wrong isn’t the body types- its the inherent mental advantage and power you have over others with it, alongside the potential harm it can do to a person’s growth. I will admit that it’s diminished by him being in a child’s body that has no authority over her. But it’s still not right. I posit that yes, he shouldn’t have sex with someone until he’s an adult, and that Roxy is a far better match for his true age than the children he would perv on.
You don’t have to fuck. It’s great and good for your mental, but it’s not more valuable than not being a pedophile.
I acknowledge the crudeness helps attract attention to the series, but it still does not make it right.
I will be a “mouth breather”, as you say, and posit that the problem isn’t his perversion, but the series inability to fully grasp the wrongness of it, and acknowledge that. You don’t need some fairy tale with a moral, but it is troubling when it doesn’t seem like the creators would have a problem with an adult passing as a kid having sex with kids.
You say the first paragraph is if I don’t agree with the idea that you shouldn’t have sex with people who are considerably dumber than you. It’s generally a little icky if you have sex with someone who is mentally deficient.
You can’t really solve the problem of isekai protagonist not being able to have sex. But when you can’t have sex, it doesn’t give free license to have sex with children.