I don't know why "slippery slope" arguments are considered a logical fallacy. It is a property of human institutions that movement in some direction makes movement further in that direction more likely. Saying it will definitely happen is still fallacious, but saying it won't make it more likely seems to ignore human tendencies in both individual and group decision-making.
One amusing non-example of this: legal far marriage in MA led to legal sibling marriage in MA.
I've heard the claim that legalizing gay marriage would lead to legalizing incestuous marriage. I strenuously doubt that it would lead to an actual conscious choice to do so. The arguments against incestuous relationships (that they are likely to be abusive due to power dynamics and the inability to separate families via devorce) are totally separate from the arguments against gay relationships (I cannot actually think of any non-specious ones)
It is amusing to note that Massachusetts, has a law making it illegal to marry your sibling, parent, child, etc. so long as they are the opposite gender as you. When in 2004 gay marriage became legal, the law this failed to prohibit same-sex marriage between blood relatives. This wasn't a policy decision by anyone: The legislature simply failed to update the code.
I find it funny that the obvious gay marriage slippery-slope is seldom discussed by its opponents: polygamy and group marriage, which have far more potential for societal impact than gay marriage ever could. I suspect this is at least partially due to the tendency of old-school religious affiliation among the small number of practicing polygamists, which the conservative "values voters" are uncomfortable associating with, even indirectly.
(Not claiming marriage equality would lead to plural marriage; just surprising how seldom it's brought up, relative to nonsense about marrying dogs.)
News issues tend to work in a sort of priority queue, where only the top three or so get much time in the news cycle. Currently, in American media, the topics are the mid-term elections, Ebola and Islamic State (mixed with conflict in the Middle East in general). Gay marriage just got pushed aside and will probably come back once the election news subsides.
Legally-recognized polyamorous relationships, in one form or another, is probably going to be the next evolution of marriage laws, and there's a bit of discussion about it, but it won't get much attention until it bubbles up to the top of the queue. That might take on the order of years or decades to happen.
While it does appear here and there, I think it's because the vast majority of gay relationships take place in a monogamous/monogamish framework, so plural marriages aren't a direct endpoint.
Whereas, "sure collect everyone's correspondences, but delete people who are citizens" isn't a far jump to "ok, you don't really need to delete anything".
I think saying it will definitely happen is pretty widely accepted within social psychology. This is called group polarization, and it's the default state that you'll hit without very, very involved countermeasures: https://en.wikipedia.org/wiki/Group_polarization
The wikipedia article is a very good summary of the phenomenon and the several factors that have a role in creating it.
It's only a fallacy if there is no basis for the caustivie chain that results in the effect we find objectionable. That is to say, if there is no clear way to get from point A to point B, it's a fallacy. If the chain of causility from point A to point B is quite clear, it's not.
It's also not a fallacy when an extensive history of similar encroachment exists. At some point, the burden of proof has to shift to the party who maintains that the government won't eventually abuse a given power.
IMHO there is no more certain indication that someone has been educated beyond their intelligence than when they sign onto an Internet forum and start braying about how the slippery-slope argument is a "logical fallacy." While technically correct, they betray an ignorance of how human beings -- a decidedly non-logical species -- actually behave.
