"You will marry Lucy," he says carefully. "Explain to me what you mean. No, wait, rather don't explain. This is not something I want to hear. This is not how we do things." We: he is on the point of saying, We Westerners. (22.80-81)
This is a classic "Us versus Them" moment, and it plays out through the contrast between city and country ways. The proposition that Petrus will take Lucy on as his third wife seems to be a totally reasonable idea to Petrus, but it is completely absurd to David.