The trend, in the present at least, seems to be that progressives introduce a previously hated concept into political discourse (gay marriage, legal marijuana), which start getting more popular the more they're debated and conservatives move on to other topics once their acceptance is just too high.
In the future, the debate about marriage may be one on the legal ramifications of a polyamorous union.
People will claim "slippery slope" in regards to that, but it is a real possibility. Looking at history there is no reason to think that things will end with the issues of today. We can pretty much guarantee that progressivism will in time move on to causes that are universally condemned in 2019, as it has done in the past. What those causes are is anyone's guess.
Once upon a time, not that long ago in fact, paedophilia was socially acceptable. And now it's not. So it's kind of cherry picking to suggest the trend is entirely in one direction.
