I don't believe either use-case is a conditional. There are no condition clauses in these sentences. A conditional use would involve a condition clause (if, had, unless, were, etc), such as:
Had it not been for the civil liberties movements of the twentieth century, legalisation of gay marriage in the US would have been quite unthinkable.
In your example sentences, the modal verb "would" is used to express a habit at a different time. Here is some more information along with examples:
"We thought they would have got home by five o'clock, but there was no reply when we phoned."
Expression of habitual aspect in past time, as in "Back then, I would eat early and would walk to school"
Essentially, you are expressing what was the norm/habit at a point in the past. "Would have" expresses a continued state of something. There is no condition here.
Acceptance of gay marriage would have been unthinkable a generation ago.
This was status quo at that point. We are not conditioning this statement on any other fact.
Generally-speaking, the choice between "would have been", "used to be" and "was" depends on context, and is style-driven. With a word like "unthinkable" in particular, some may not see a distinction between the three. However, to get a sense of tonal difference, contrast with the following sentence:
Even in the previous century, segregating priests of colour was unthinkable.
Here, the emphasis is on a single case that didn't fit the norm, so we use "was" instead of "would have been" or "used to be".