Why does Islam teach us to marry our daughter-in-laws?

The Koran tells us to emulate the prophet. Well prophet Muhammad lusted after his son's wife. His son had fallen in love with a girl and the prophet initially gave him his blessings..... but changed his mind when he saw her..... his son had only been married to his new wife for a few weeks when the "prophet" told him to divorce her asap because of a "revelation" which said that she was destined to be a wife of the prophet....... then he married her against her will