This makes me a little more confident that you are starting from your conclusion based on your individual morals/policy preferences and then coming up with the argument that's not clearly inconsistent with the text of the constitution. Which is understandable and the way most people think it should work (or at least want it to work when the person doing the deciding agrees with them). But I think people only think that because nobody has approached the constitution that way from a rightist perspective, and only Kennedy has approached it that way from a libertarianish perspective. If we had four supreme court justices trying to claim that capital gains taxes violate equal protection because they tax future consumption more heavily than current consumption, or that progressive taxation in general is suspect under the equal protection clause, you'd see a lot more people understand that it's a bad idea to not be bound by any originalism, but free to make whatever arguments are not clearly contradicted by the text of the constitution.
I'm trying to see your perspective here, but your examples aren't helping. The tax laws are applied fairly and equally to all Americans so I'm not sure what this has to do with the Equal Protection clause.
The problem that comes up with originalism is that when you're trying to come up with general protections that will be relevant for centuries to come, you are going to necessarily write something that is generic so as to cover ideas and cases that you have no idea about. How do we apply freedoms of speech and privacy to modern cell phones? The original authors had no idea that these things would exist, but we still use originalist arguments to say that "well, their intention in this matter would be..." and you can more or less make up whatever you want as long as it sounds good enough in your head. And what are you going to argue for? Your natural tendencies and biases.
So you've been arguing that the 14th obviously can't extend to protect other classes because it was written for a specific purpose, and that since we had to ratify the 19th, obviously an idea like the 19th amendment is not covered in the 14th so you can't extend the 14th to include things like homosexual marriages.
Except the 19th was written not because the 14th was deficient, but because our congressmen were deficient. We can obviously see that things such as Japanese Internment are obvious violations of the 14th amendment, but they still happen anyways. The original authors' purpose can and should be bigger than simply the original purpose of their writing.
So when you've arbitrarily decided that only straight people can get married, and thus collect a huge a tax benefit, and gay people can't (and not to mention the sodomy laws still on the books). How can you call that anything but denial of equal protection?
I think you're trying to find a hypothetical situation that would be an obvious overreach of the 14th, but I don't think you're succeeding. If I understand correctly, you're saying that without originalism, you could claim a progressive tax system is unfair treatment. But how much money you earn isn't a part of who you are, and those laws are applied equally across all Americans. You're attempting a slippery slope argument:
https://en.wikipedia.org/wiki/Slippery_slopeThat is you're saying "well if you give equal protection to x, then by the same logic you have to do
this obvious thing no one would agree with" And the reason this works, is because the court is allowed to use common sense. Anyone can see that it wouldn't make sense to extend the 14th to include progressive tax systems (and the court has already decided the constitutionality of progressive taxes.) I'm sure you could make wild claims about certain conclusions you could make from originalism, but no here is going to present them because they would equally be a slippery slope fallacy.