LG # 25 says that a position is part of the ordinary infallible magisterium if the teaching has been taught as a "definitive" part of Catholic teaching (not that the bishops and Pope have to get together and say "this is definitive--we have taught it for a couple of centuries but we REALLY mean it" -- that would be a quasi-extraordinary definition that even some of our Trinitarian and Christological formulations have not been subject to--and yet nobody would say they are open to dissent in conscience by good Catholics).  The Church's teaching on contraception has been consistently taught by ALL Christians until 1931; it was taught definitively by the Church since, affirmed in Humanae vitae, reaffirmed in Evangelium vitae, repeatedly repeated by John Paul II, Benedict ....  It is ordinary infallible teaching.