Something I have heard quite a bit about lately, at least in public discussion, is the abusive accusations of the Slippery Slope Fallacy. The proper understanding of this logical fallacy is when someone argues that the consequences of accepting one point will inevitably lead to a series of events leading to an unacceptable conclusion without any real logical or empirical evidence that one necessarily leads to the next:

If a then b, if b then c, if c then undesirable d.

Indeed, this type of logic is fallacious, but it is often confused with the reductio ad absurdum, where one attempts to show that the consequences of accepting certain premises (not conclusions) logically implies something which is very obviously absurd or unacceptable. That is, to make the following claim:

If a then b, but if a then also undesirable c and d.

This sort of confusion occurs the most, it would seem to me, when discussing matters of sexual ethics. It is common for those who wish to morally justify certain behaviors to define moral parameters & premises from which they can achieve the desired outcome. However, inadvertently they end up also justifying a slew of other behaviors which (at least at the moment in our current cultural context) seem utterly reprehensible. Yet when this is pointed out the accusation tends to be thrown of committing the Slippery Slope Fallacy.

For example, when one accepts the conclusion that the use of contraception is morally licit based on the premises that sex does not have the primary final cause of procreation, but rather it's simply a matter of pleasure, of which the only limiting criteria is the vague notion of "consenting adults," these premises logically allow for all sorts of depraved behavior ranging from fornication, to sodomy, to incest, and beyond. This is not a matter of the Slippery Slope Fallacy, but that the parameters used to define the moral limits of the act itself are too loose and do not establish a coherent model. Therefore, if someone were to retort to the acceptance of contraception under these premises that such logic would also justify many behaviors we would most certainly condemn, they engaging in a reductio ad absurdum, not the Slippery Slope Fallacy.

Of course, how this looks at a social level will end up being different from the pure logical parameters of the acceptance of certain conclusions. Most of us pass our judgments not based upon logical statements & conclusions, but on intuition and what we consider to be normal. This is not necessarily problematic as we do not generally have the time to do all the logical calculations for everything in our life, otherwise we would never get out of bed in the morning. Yet the premises of these logical statements formed by the social philosophy of our time do eventually get their conclusions to be normalized over time, albeit slowly. Hence, after the acceptance of contraception we did not automatically see also the acceptance of all the other conclusions of the aforementioned logical statement.