It is not Pius XII's words to midwives that we are talking about here, but the words he put in the Acta Apostolicae Sedis (which happen to be the same words) that are the point. When a pope teaches the entire Church (usually through the Acta) on a question of faith or morals (which this is), Catholics are required to accept it.
Moreover, I am not aware of any Catholic theologian who has ever said that NFP is a form of contraception. If your position were correct, there would be dozens. Maybe you could give us just five or ten quotes for now, though. :)
Not everything in AAS is of the same authority. You do realize that, right? You guys treat this speech to midwives, and, yes, that's all it was, not an Encyclical, not addressed as a teaching to the Universal Church ... as if it were in the same category as a solemn dogmatic definition, infallible, irreformable. Msgr. Fenton addresses that even within Encyclicals, you have to look at the language to determine whether a pope is intending to teach something with his authority. Not every word is intended to be taught to the faithful. You could have a Pope go on for 20,000 words in some Encyclical, with various explanations and thoughts, but then there might be one or two points he's actually attempting to teach. Not all of these
obiter dicta are even authoritative, much less infallible. It's clear that Pius XI was teaching to the Universal Church with his authority in
Casti Conubii, and it's clear from the language where he's teaching something and where he's engaging in explanations. Unfortunately, too many SVs feel that if it's in AAS, then it constitutes something that's being taught to the Church, every word of it, and might as well be a solemn definition. You should read Msgr. Fenton's essay on the authority of Encyclicals where he goes through all the distinctions.