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Description of Slippery Slope

The Slippery Slope is a fallacy in which a person asserts that some event must inevitably follow from another without any argument for the inevitability of the event in question. In most cases, there are a series of steps or gradations between one event and the one in question and no reason is given as to why the intervening steps or gradations will simply be bypassed. This "argument" has the following form:

   1. Event X has occurred (or will or might occur).
   2. Therefore event Y will inevitably happen.

This sort of "reasoning" is fallacious because there is no reason to believe that one event must inevitably follow from another without an argument for such a claim. This is especially clear in cases in which there is a significant number of steps or gradations between one event and another.
Examples of Slippery Slope

   1. "We have to stop the tuition increase! The next thing you know, they'll be charging $40,000 a semester!"

   2. "The US shouldn't get involved militarily in other countries. Once the government sends in a few troops, it will then send in thousands to die."

   3. "You can never give anyone a break. If you do, they'll walk all over you."

   4. "We've got to stop them from banning pornography. Once they start banning one form of literature, they will never stop. Next thing you know, they will be burning all the books!"

Actually SJB, it's not.  Since Antipope Pius XII not only never defined "grave motives," but even painted a very broad and vague picture of what those "may be" and "are" he left the floodgate open for such as we see nowadays, namely that all sorts of things are being called grave motives, where truly there is no such thing as a sufficiently grave motive that would allow couples to continuously engage in their lusts for one another while attempting to prevent conception.  That is birth control, plain and simple.

That is a "slippery slope" argument. Did the Church's own moral theologians have anything to say about "grave motives"?

...continuously engage in their lusts for one another while attempting to prevent conception.

Have you ever read any moral theologians on this issue? What do they say?

And was Pius XI also wrong here, in principle:

59. Holy Church knows well that not infrequently one of the parties is sinned against rather than sinning, when for a grave cause he or she reluctantly allows the perversion of the right order. In such a case, there is no sin, provided that, mindful of the law of charity, he or she does not neglect to seek to dissuade and to deter the partner from sin. Nor are those considered as acting against nature who in the married state use their right in the proper manner although on account of natural reasons either of time or of certain defects, new life cannot be brought forth. For in matrimony as well as in the use of the matrimonial rights there are also secondary ends, such as mutual aid, the cultivating of mutual love, and the quieting of concupiscence which husband and wife are not forbidden to consider so long as they are subordinated to the primary end and so long as the intrinsic nature of the act is preserved.

Are you saying that the subordination of the secondary ends to the primary ends TOTALLY negates the secondary ends? Did Pius XI just not see this?