CathInfo.com

It is a matter of fact-- not opinion-- that the Holy Offices of Pope Pius IX (1853), Pope Leo XIII (1880), Pope Pius XI (1932), and of course Pope Pius XII, all taught that periodic continence can be lawful.  It is also a matter of fact-- not opinion-- that during that hundred year period (1853-Pius XII), theologians unanimously agreed on its intrinsic lawfulness (although they did disagree on some of the upstream principles and downstream applications).  It is also a matter of fact-- not opinion-- that theologians were never rebuked for believing it to be a lawful practice.
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Given the facts, I would agree that the Church infallibly teaches that periodic continence can be lawful to use.  The alternative is to suppose that a hundred years worth of popes and theologians universally erred on the matter.  Let's keep in mind who those popes are-- not just Pius XII (as it is so often ignorantly alleged), but Pope Pius IX and Pope Leo XIII.  Pope St. Pius X, a militant pope on moral and doctrinal matters, did not censure or rebuke theologians for teaching it.  Pope Pius XI is included among these popes as well (which should give anyone who thinks that Casti Connubii denies the lawfulness of periodic continence pause, since no sooner was the ink dry on that encyclical than did the same pope explicitly confirm its lawfulness via the Holy Office).  The scope of periodic continence being taught exceeds the scope of religious liberty being taught both in time and space. 
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Of course, it is one thing to simply reserve judgment on whether or not this amounts to infallible teaching; it is another thing altogether to accuse such orthodox men, over such a long period of time, as having taught grave or even heretical moral theology. 
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Importantly, I should point out that an expression like "the Church teaches periodic continence" should be understood to simply mean that the Church teaches it can be lawful to use.  The Novus Ordo seems by all accounts to teach something very different, they seem to teach that it ought to be used in some general away.  The Catholic Church has never taught that, she has merely taught that there is nothing intrinsically evil about it, and regularly warned that it is something that should be taken seriously, not taught (as a method) publicly or indiscriminately, and only used when there is a commensurate reason.
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Yes.  NFP is not intrinsically evil as is straight out contraception.

That's not correct.  What is not intrinsically evil is abstaining from marital relations.  But the formal motive is what makes it evil ... and what turns mere abstinence into NFP.  Evil is evil, and sin is sin.  If someone abstains only during a certain time in order to avoid conception, the formal intent there is to subordinate the primary ends of marriage to the secondary ... which Pope Pius XI denounced as sinful in his encyclical Casti Conubii.

That's not correct.  What is not intrinsically evil is abstaining from marital relations.  But the formal motive is what makes it evil ... and what turns mere abstinence into NFP.  Evil is evil, and sin is sin.  If someone abstains only during a certain time in order to avoid conception, the formal intent there is to subordinate the primary ends of marriage to the secondary ... which Pope Pius XI denounced as sinful in his encyclical Casti Conubii.

Lad, it is fitting that you should bring up that argument now, since we are nearly at the one year anniversary of me having argued against you that Casti Connubii contains no such denouncement.  Has a year been long enough for you to develop a response to my criticism? :)