Owning slaves is all about the social position in which Wang Lung wants to live. He buys himself female slaves, a direct insult to his wife who was once just that, and yet when his son begins to mature and become interested in women sexually..... he won't do the same for him. I don't think Wang Lung is a bad man, he's just lost his perspective on what a "good" man is, and his priorities have been completely mixed up. He isn't happy with just having the money, the house, the slaves, the concubines, and the nice nice clothing; he want the prestige of those things.