He'd hesitated and then Nadayaki seemed to be trying to take charge. It's fine. Especially the nibbling. He arches his neck away, then goes with the roll. Once there, he gives Nadayaki's neck a flick of his tongue, rubs his back muscles, then reaches around underneath. As far as he knows, even especially humanoid (mutantoid) aliens might have very different genitalia. Can't blame a man for being curious.