How would you analyze David and Lucy's relationship in the novel Disgrace by J.M. Coetzee?

David feels that Lucy is not safe alone on the farm and keeps insisting that she move away. Lucy will not moveand abandon her way of life. Petrus has proposed marriage to Lucy. This seems ridiculous, since Petrus already has two wives, and the only reason he wants to marry Lucy is to take her property. Lucy sees this as an option, as a way for her to live more safely on the farm.