I was unaware that there was ever an infallible statement made on the use of contraception. Doesn't the pope have to proclaim a teaching ex cathedra before it becomes an infallible teaching?

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.

 Lumen Gentium (25) says:

This religious submission of mind and will must be shown in a special way to the authentic magisterium of the Roman Pontiff, even when he is not speaking ex cathedra; that is, it must be shown in such a way that his supreme magisterium is acknowledged with reverence, the judgments made by him are sincerely adhered to, according to his manifest mind and will. His mind and will in the matter may be known either from the character of the documents, from his frequent repetition of the same doctrine, or from his manner of speaking.