It could be considered a fallacy due to arguments saying that "this will lead to this which will lead to..." and so on. While some of these options are statistically possible, that does not always justify assumptions on a large scale. e.g. saying that wearing black will lead to the decline of human
It is a fallacy when it is used fallaciously. While a strongly supported causal link would still define it as a slippery slope argument, it would not make the argument fallacious. Any form of causal linking, however, that lacks incredibly strong support to back it up, is fallacy.
Society progresses with velocity. If a drug is decriminalised, there is a greater chance it will be legalised. If society becomes more tolerant of an abundant sexuality, it will more likely tolerate less common sexualities.
"If society becomes more tolerant of an abundant sexuality, it will more likely tolerate less common sexualities." This, however, is a SS fallacy because there is a wealth of evidence to the contrary—that societies that are more sexually permissive tend to have fewer dangerous sexual proclivities.