Keep reading. If Peters does not deal directly with the doctrinal issue, he refers to people who do.
Discipline is always based on doctrine. Discipline that is not necessitated by doctrine is a mere imposition. (And the Church is vigilant not to impose disciplines on the faithful that are not logically necessitated by doctrine.)
Arthur, I don’t think you’d want us to conclude that the 21 other Churches in communion with Rome (Melkite, Maronite, etc.) which to not require clerical celibacy, are doctrinally deficient. Do you have an answer which would take them into consideration?
Just found this which you and other Catholics might find interesting:
http://www.unamsanctamcatholicam.com/history/79-history/465-celibacy-in-the-early-church.html
So what doctrine mandates celibacy for the priesthood, the forbidding of priests to marry, or making them leave the priesthood if they do, or prohibiting married men from becoming priests unless they're transplants from other denominations?