ALL??
Not a majority?
It's not a vote. A Pope can issue various types of doctrinal statements, but they have to be in unity with the bishops working with him, and with the Deposit of Faith, which includes the Bible and all preceding doctrinal teachings of the Church stemming from the Bible. Any papal statements of doctrine are usually co-signed by the heads of pertinent groups of bishops, such as the Congregation for the Doctrine of the Faith, the various councils and so on. Hence, a pope couldn't just overturn the meaning of marriage as a sacrament or the Biblical teachings that lust, fornication and homosexual sex are sinful. It wouldn't be valid.