It's a game he plays with your wife, placing just the tip, just the head of his cock in her pussy, stopping, waiting. He knows she wants is, wants him to push it into her, to fill her. He knows she's conflicted, sometimes ashamed how badly she wants cock, how much she needs it.
He knows she tells herself that when he pushes into her she's not really cheating. Not really doing anything wrong. After all, he's the one fucking her, not the other way around.
So he teases her. He waits. And waits. And waits.
Eventually, she can't take it anymore and starts moving backwards, slowly impaling herself on his thick cock.
He waits because he knows when she does it, she's admitting it, that married or not, deep down inside she's a slut. Deep down inside, no matter how much she loves her beta husband, she craves cock. No matter how prim and proper, no matter how vanilla her sex life at home, what she wants, what she needs is cock.