The Slippery Slope is generally considered to be a fallacy. It is an argument in which a person asserts that some event must inevitably follow from another, prior event without any providing any argument for the inevitability of the event in question. In most cases, there are a series of steps or gradations between the eventual event and the one being argued against but no reason is given as to why the intervening steps or gradations will simply be bypassed or enacted in their turn.
In general I agree. The Slippery Slope is a fallacy and, much more often than not, sloppy thinking. But what if we’re greasing the slope as it were?
Many laws, especially those dealing with sexual partnering, sexual acts, and sexual relationships are wholly or primarily based upon societal mores and cultural “prejudices.” Many of those laws that once had valid scientific rationales at one time have lost those rationals due to medical advances. Others had and still have outcome-based basis in the effects such things have upon the titular children of such unions, yet those negative consequences have been ignored in favor of a new ideology.
As you chip away at those laws that are based upon societal mores and cultural “prejudices” or apply new societal mores and cultural “prejudices” to existing situations you can make it so that each increase in permissiveness makes the next incremental increase that much more likely through normalizing or “main streaming” what was once unthought-of, abhorrent, and/or outre.
Western, especially American, jurisprudence also has to be taken into account because it’s inherently based upon Common Law and under the US interpretation of such court cases are decided based upon prior argument, precedents, and judicial pronouncements rather than solely on legislative enactments. in other words, arguments and verdicts in prior, similar court cases can be compelling or even binding precedents in current and future cases.
The Slope As It Was, Is, and Will Probably Be
Progress Is Opinion. Progression Is Mathematics
The battle to overturn the laws against miscegenation was long and hard-fought but it was victorious. But many of the same arguments that were successfully used Loving v. Virginia to end those “Jim Crow” statutes were later used in Lawrence v. Texas to abolish or make largely enforceable the various laws against: Sodomy, Fornication, Adultery, and violent sexual fetishism (BDSM). In turn, arguments from both Loving and Lawrence have been used to legalize Gay Marriage in various states and to overturn DOMA at the federal level.
Each set of court cases added their levels of grease to the slope, making it increasingly slippery. Hence, there is no logical reason not to predict that this progression will continue and that these increasingly entrenched arguments and precedents will be used to legalizing polygamy, incest, and some forms and/or levels of pedophilia.