She denies him sex and humiliates him - is it cruel or is it love?

They are a happy couple, deeply in love, and have enjoyed decades of marriage together. From the outside they seem like the couple next door - normal in every way. But there's one thing, their sex life looks quite different than the usual sex life of couples. She denies him PIV sex and taunts and teases (humiliates) him about it. Isn't humiliation cruel and damaging to a relationship? Why would a husband be happy and fulfilled being denied sex from his wife? Wouldn't she lose respect for her ...