Having laws that permitted slavery did not give any moral legitimacy to pretending that one man could be the property of another man. Similarly, having laws that permit gay “marriage” does not give any moral legitimacy to pretending that one man could be the spouse of another man. They seem to be winning the battle to pretend that these relationships are marriage, but changing the words does not change reality no matter how much the far left wishes that would work.
Back when slavery/segregation were legal and court sanctioned, they were still obviously immoral, seeing a human being whipped or hanging from a tree produces a visceral reaction which is hard to explain away. Homosexual right are more complex because they come with a whole worldview that justifies this as the moral thing to support while smearing the moral opposition as immoral itself. Alot more devious and hard to oppose.