A) over a thousand years of Catholic tradition. B) the decree of the Magisterium.
I don't pick and choose what I will believe. At every Mass I attend I affirm my belief in "the One Holy Catholic Apostolic Church". "Apostolic", as in: I trust the apostolic succession of authority. I trust the continuity of the faith as handing down by the apostles through the bishops and the councils and the See of St. Peter himself. Who in the hell am I to question such authority? Who are you to question it? If you do think you can question it, are you aware that you are breaking an oath every time you affirm your loyalty and belief in the "the One Holy Catholic Apostolic Church" during the Mass?
Mandatory celibacy for priests is a discipline, not a doctrine. No one is required to believe that priests have to be unmarried at peril of their soul.
A celibate Priesthood is a matter of discipline, not doctrine.
Provided one upholds the right of Rome to restrict the priesthood as it sees fit, and follows the law as in effect, one can certainly advocate that the discipline be changed without becoming a non-Catholic.
There are many worse things that could happen to the Church or have already happened to the Church than allowing married men to be Priests.