The 14th amendment is a slavery amendment and nothing more.
Then why are the judges basing all of these decisions on the 14th Amendment?
I don't suppose they'd use, say, the 21st as a grab bag for their judicial activist fantasies.
Convenience.
It's not just that it has wording designed originally with slavery(former slaves) in mind, but it's also due to the fact that [at least in my experience] the idea of applying originalism to the 14th amendment is one of the most unpopular ideas in the United States.
Progressives most certainly don't want to be all originalist with the 14th amendment.
Here is a clue. Most conservatives don't want to be all about the originalism with the 14th either. I don't see it. Typically, originalism is only supposed to apply to the first authoring of the Constitution, and perhaps by extension encapsulated by the inclusion of those amendments comprising the Bill of Rights. Originalism for some odd reason ends there.
Originalism could have put an end to a lot of our problems decades ago. It's not hard to go reading the debate notes from when they created the 14th amendment. Guess how many times in 1865 they discussed gay marriage, for example?