Did Mary Have Other Children?

All of us are probably aware that the Catholic and Orthodox churches believe and teach that Jesus was the only child born to Mary. In 411, Augustine wrote that Mary, “remained a virgin in conceiving her Son, a virgin in giving birth to him, a virgin in carrying him, a virgin in nursing him at her breast, always a virgin” (Sermons 186, 1: PL 38, 999 as quoted in paragraph 510 of the Catechism of the Catholic Church). We might also know that many Anglicans believe this. But we might be surprised that some Protestants believe this. In fact, Martin Luther, Huldrych (or Ulrich) Zwingli, Heinrich Bullinger, and John Calvin all believed it. In his Commentary on Matthew, Calvin even called anyone who disagreed “pigheaded and fatuous.” Perhaps even more surprising is that the eminent Baptist theologian, John Gill, regularly waffled on this subject, seeming to believe it by comments he made, but never, as far as I know, making a clear statement committing himself to it. Is there clear, biblical proof one way or the other?