It's told from a male's point of view. In it, he starts dating a woman. As much as he finds her attractive, he likes the idea of her getting it on with her son even more. He plants the incest seed in the son's mind first, who with little or no remorse agrees. The mother is the tough part; she shocked and revolted at first, but the man gradually breaks down her defenses. I thought this was a really hot story, but I can't remember anything more than what I've given here. If this story rings a bell with anyone, please help me find it.