To be fair, I doubt those who do said sin are focusing on their hand.
And to his point, the brain is especially elastic where it concerns sexuality. I am not sure what the extremes of said elasticity would be, but I can imagine a reverse Pavlov Dog’s effect at least a little taking place if one were actively pursuing it. And, the demons, in seeing this perversity, may assist him in his descent to make it all the more easier. Though, one may question if the man was willing to do all this, what his disposition was at the beginning of the thought experiment.
Also, in response to another comment, God is One Being in Three Persons.