He said that he's impotent, specifically. However, that doesn't answer the big question. Men that are impotent still have sexual desire and lust, they just can't get hard-ons. This is an important detail, because it means that if he had the same level of desire as the rest of his species, then his first goal would have been to regain the ability to fuck, which he would have achieved much more easily than everything else he did.
On top of that, everything that was mixed with goblins resulted in excessively horny creatures no matter what he did to them, which makes him an extreme anomaly. He observed it himself, that education and reasoning changed nothing. So it wasn't his education that made him different, there was something fundamentally different about him from step one.
It makes me wonder if the dev actually thought that far, about what's going on there, or if it's just an unresolved mystery that mainly comes from me thinking too hard about a hentai game. XD