Well ... it's probably not an exercise of the "extraordinary magisterium," but it's without question infallibly known. JP2 said this (Evangelium Vitae 62):
Given such unanimity in the doctrinal and disciplinary tradition of the Church, Paul VI was able to declare that this tradition is unchanged and unchangeable. Therefore, by the authority which Christ conferred upon Peter and his Successors, in communion with the Bishops-who on various occasions have condemned abortion and who in the aforementioned consultation, albeit dispersed throughout the world, have shown unanimous agreement concerning this doctrine-I declare that direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being. This doctrine is based upon the natural law and upon the written Word of God, is transmitted by the Church's Tradition and taught by the ordinary and universal Magisterium.
I was thinking along these lines. I just finished reading a book that covers the canonization of Pius IX and Vatican I (the Papal Infallibility issue), Pius had asserted Papal Infallibility prior to Vatican I in declaring the Immaculate Conception a dogma of the faith. Apparently most of the council fathers believed that on all pronounced matter of faith and morals, papal infallibility was assumed.
The quote above by JPII, appears to invoke Papal Infallibility, therefore being pro-abortion is NOT an option for Catholics. Am I wrong?
BTW, I have never been Catholic (although I did attend Catholic school as a child), but I am a Catholic sympathizer/supporter (LOVED JPII). Also, I am a religious historian (Medievalist), which means my research area is pretty much Catholic History (and the medieval heresies).