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".
(Not claiming marriage equality would lead to plural marriage; just surprising how seldom it's brought up, relative to nonsense about marrying dogs.)