I think that slippery slope can definitely be an indicator of a problem. I've used this a lot myself.
Pius XII opened the door to NFP. Now NFP has become so pervasive that it's effectively Catholic birth control. Even if Pius XII's teaching did not necessarily, if properly undestood, logically lead to the latter, there was something wrong or imprecise or defective about the original teaching that allowed people to run with it. So, while this slippery slope, doesn't per se prove anything, it's definitely an indicator of a problem that must be examined.
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Well that's bait if I've ever seen it.
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Pius
IX opened the door to NFP when his Holy Office approved of its use the first time the Church was ever questioned about it in 1853. Then Leo XIII did the same thing with his Holy Office, then Pius XI did the same thing with his, then Pius XII the same thing with his, all the while every other pope (including Pope St. Pius X) supervised and failed to rebuke a
century worth of theologians who all taught its lawfulness. The Onigo-Knox system disseminated by Dr. Latz out of Chicago has a
far more proximate convergence with the abuse of NFP than the Church's teaching did... the Church was approving of it for 75 years and it didn't start to become contentious until medical scholars finally "figured it out."
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Proximity is key if one is going to try to make value of a real slippery slope. Exactly how proximate does a have to be from b, I don't know. But NFP is a good example... it was approved without any social discord for almost a hundred years, and once the social discord
did arise, it arose almost
immediately after the publication and dissemination of Dr. Latz's work which synthesized the literature on Rhythm and disseminated it for a popular audience with charts, instructions, etc. on how to use it. Not to sound like I'm giving a
post hoc argument, only to point out that if we're starting at a high level and trying to pinpoint a problem, proximity is going to need to guide us. Otherwise we might as well just distill everything to the fall and call it a day.