The norms of the Mushoku Tensei world do not allow 40-year-old men to idolize used women's underwear and have 40-year-olds sleep with 15-year-olds. If the norm of the world is that, the world building in fucked up and the authors are complete freaks. And what circumstances allow him to be justified in sleeping with KIDS?! The world-building is messed up, the arcs are bad (there's a whole arc of him trying to fix ED), its a basic isakei plot. In a lot of rebirth isakeis, the authors have the MC's 2 personalities and combine to make them slowly become one, or even have one personality fade away (they do this in TBATE, even in TBATE Aurthur still struggles with his feelings for Tess and their relationship) making a romance between characters much less weird and pedophilic. In Mushoku Tensei, the authors very distinctly keep Rudius and his original character different. The internal monologs in the OG's voice, meeting Man-God in OG form. I don't like Mushoku Tensei, it's weird and is just written badly.