> The concept of marriage can't be dismantled, at best we could remove any state concept of it and get rid of any legal protections, tax benefits, etc
In Australia, unmarried couples (what we call de facto relationships) have essentially the same legal rights and benefits as legally married couples. In fact, Australia extended de facto status to same-sex couples before it legalised same-sex marriage, rendering the latter move an essentially symbolic measure, at least as far as Australian domestic law goes. There's no reason why other countries (including the US) couldn't do the same thing, except maybe "conservatism"?
And once unmarried relationships are made legally equivalent to married ones, you don't need to retain government recognition of marriage. One could just repeal marriage laws, and abolish marriage as a secular legal concept. If individuals want to get married as a cultural or religious tradition, that's a private matter in which the government doesn't need to get involved. If a couple are separating and can't agree on issues such as property and children, and hence need the courts to decide those issues, the courts don't need to know or care whether the couple are "married" or not.
I think most gay liberationists would be happy with an outcome in which marriage disappears from the law, and starts to fade from mainstream culture. Yes, there will probably always be minorities of religious conservatives/etc who retain it, but there's a saying "don't let the perfect be the enemy of the good"
> This is a really strange view on language in my opinion. We absolutely could define what the terms conservative and progressive mean, and the definitions could be absolute rather than relative
You can define words to mean whatever you want. The problem is, different people define words differently, and if definitions disagree, what makes one person's definitions objectively superior to another's?
No doubt one can come up with completely unreasonable definitions – if someone was to define either "conservative" or "progressive" as "the belief that the moon is made of green cheese", that's obviously not a definition worthy of anyone's time. But, if radical progressives start arguing "'mainstream' progressivism is really conservatism", that doesn't seem to me to be an inherently unreasonable position, in the way that the 'green cheese' definition is. It is a reasonable definition if their views are right; and its value if their views are wrong may depend on how exactly one thinks their views are wrong.