Concerning the infallibility question, the pope doesn’t need to proclaim ex cathedra the teaching that artificial means of birth control is wrong; it’s already an infallible teaching of the Church by virtue of the ordinary and universal magisterium.
This loyal submission of the will and intellect must be given, in a special way, to the authentic authority of the Roman pontiff, even when he does not speak ex cathedra, in such wise, indeed, that his supreme teaching authority be acknowledged with respect and sincere assent be given to decisions made by him. (Lumen Gentium 25)
For further clarification on this issue, please see "What Is the Magisterium?" by Thomas Storck at www.catholic.net.