At what point, and why, are priests not to marry?

All throughout history, the rabbis married and had children. Peter was married, and he was one of the founders of the church, and is considered by Catholics to be the first pope. So why, and when, did it become a rule that priests couldn't marry?

Catholic priests were allowed to marry for centuries. For a few more centuries they were "allowed" to, but it was frowned upon. It finally became outright not allowed because the church got tired of priests dying and leaving their land and property to their family rather than to the church. So they did away with that so that priests wouldn't have a wife or children to leave their land to so they'd leave it to the church.