I know people don't like the "Rhythm Method" for it tends to be unreliable, but since 1939 the Catholic Church has said sex is for more than procreation (The ruling was made by Pius XII). Pius XII expanded on this in two letter in the 1950s.
Now Pius XI, Pius XII predecessor had been more anti-nazi than Pius XII was. Part of this is Pius XI died in 1939, before WWI began, and Pius XII was pope during the fighting and wanted to stay neutral as much as possible (More in the hope of negotiating a settlement as Pope Benedict XVI had tried during WWI). Pius XI read a book written by an American Jesuit attacking segregation in the American South. Pius XI approved of the book and asked the writer to write a similar paper on the Nazis and the Jews. Pius XI also issued a statement stating "we are all Semites" as Hitler made the life of the Jews in Germany worse.
Now both, Pius XI and pius XII had the same name, but how they viewed the Papacy was different. Pius XI was a book worm, read extensively, was the Vatican Librarian before he became pope (Through did operate in the Diplomatic Corp of the Vatican). Pius XII had almost always served in the Diplomatic Corp of the Vatican. As such Pius XII preferred to talk and discuss problems and did NOT like do anything that may offend someone (Thus when the book about German Jews was finished when Pius XII was pope, Pius XII suppressed the book let it offend Hitler, believing that he could NOT deal with Hitler if Hitler was offended by the Book, an unimportant concern to Pius XI). Just two different men with difference in how to use the papacy, but as a whole both had the same goals, an Independent Catholic Church and a rejection of the idea that one's religious belief should be kept out of sight. Thus unlike other Religious leaders they had to address the issue of Sex, and did so by backing the "Rhythm Method". You may disagree with the birth control method, but at least they addressed the issue as opposed to ignoring it.
For more on Pius Xi:
http://en.wikipedia.org/wiki/Pope_Pius_XI http://www.vatican.va/holy_father/pius_xi/index.htmFor more on Pius XII:
http://en.wikipedia.org/wiki/Pope_Pius_XIIhttp://www.vatican.va/holy_father/pius_xii/The 1938 letter attacking Nazism:
http://www.vatican.va/holy_father/pius_xi/encyclicals/documents/hf_p-xi_enc_14031937_mit-brennender-sorge_en.htmlReports on Pius XII and the Holocaust:
http://www.catholicleague.org/pius/framemain.htmhttp://www.piusxii.info/