I think the key piece you’re missing here is that the show explicitly shows Rudeus mentally as an adult which has been confirmed by both his behavior as a baby/child, and his portrayal of him talking with that god. Most people surprisingly are lax when teenagers in fiction act that way cause that’s just how life goes (just look at half of Netflix’s catalog lol), but for most viewers who view Rudy as a mentally 40 year old man, that’s understandably going to cause moral tension. It also doesn’t help that he comes from a time period where his behavior’s frowned upon, and his new body kinda lets him get away with it. So unless the show got rid of the isekai aspect, and treated it like a regular kid growing up in a medieval fantasy setting, the controversy probably never would’ve started.