@Dobelius I think it's ok, as long as it will make a person feel more "complete" on a physical and psychical level. Problem is, most persons who do things like that focus too much on developing physically, while they don't pay the same amount of attention to some other aspects which would improve their lives, and etc
Unless you have no teeth it is logical that it would painful. A baby forces the milk out by pushing the aeriola against the roof of its mouth, if there are teeth the nipple will be rubbed against them. And cheating on your wife and children will bring you all problems later. You can never rely on how 3rd parties will behave.