I suppose that, to Vito's way of thinking, it was ok for Johnny to play around (the norm in the Mafia milieu although not for Vito), and he was more indulgent of his godson than his own son. And, he'd probably, in his way of thinking, regard the "sandwich" as a "perversion."

But, we get a hint of Vito's straightlaced character much earlier. He and Clemenza steal the carpet, then Vito loses his job at the grocery. He's in a bind because, as Puzo points out, he "couldn't eat the rug."