Huh? Hasn't Trek always been a biased mouthpiece for social agendas?
Not really, no. Star Trek has always been good at exploring complex social issues (through the veil of an alien setting), but, contrary to the popular imagination, it rarely takes a clear side even when it's doing social issues through parable, much less openly.
When Trek does go ahead and get preachy, it tends to suck. ("Let That Be Your Last Battlefield" is mostly a terrible episode, we need hardly recount the flaws of "The Omega Glory" here, and "Up The Long Ladder" is a worse dumpster fire than "Shades of Gray.") Scan any "best episodes of Star Trek" list, and you'll find a lot of amazing explorations of social issues, but few where Trek came down firmly on one side or the other of any contemporary controversial social issue. (Sometimes it will come down firmly on both sides! "Homefront" / "Paradise Lost" made effective, respectful cases for both sides of the post-9/11 civil liberties debate five years before 9/11!)