The show did seem to be progressive about broad issues but not progressive about specific, current issues. It said that humanity could come together and get past all of its current failings, but it was not bold enough to push controversial issues before they went mainstream. And when it did it addressed them in an obvious pedantic way, like that environmentalism episode or Stigma. These episodes came out in a time when a majority of the audience already agreed with them, didn't say anything new or unsafe, and usually turned out among the worst episodes in the series.
It's probably best Star Trek didn't have a gay character though. I doubt any of those writers would have been able to write a gay characters and just have it be a part of his life rather than his entire life, and then every single time they did an episode about that character it'd be entirely about his sexual orientation. Especially on Voyager and Enterprise, any gay character would have had no other personality traits than 'The gay guy'.