It is a legislative act later ratified by the states. The fact that it was later ratified does not make it any less a legislative act @Maj. Bill Martin
1) So was the Constitution itself. The Continental Congress passed it, but it then had to be ratified by 3/4 of the states - the exact same process as all of the Amendments. It was ratified on June 1, 1788, and effective March 4, 1789.
2) A Joint Resolution (not a "law") passed by both houses of the First U.S. Congress on September 25, 1789 that proposed 12 new Amendments was sent to the States pursuant to the Constitution. Ten of them were then quickly ratified by the States to become the Bill of Rights.
3. A Joint Resolution passed by both houses of the 39th Congress on June 18, 1866, proposing a new Amendments was subsequently ratified by states to become the 14th Amendment.
Your argument that the 14th Amendment is invalid because it was a "piece of legislation" makes zero sense. That's the exact process used to ratify not only the US Constitution itself, but also specified within the Constitution for all future Amendments, and was the same process used for all of our other Amendments, including the Bill of Rights.
I'm not sure how you came up with that argument, but your reasoning would invalidate every single one of our Constitution Amendments, including the entire Bill of Rights, as well as the Constitution itself.
I've read a lot of arguments against the idea that the 14th Amendment establishes who is a natural-born citizen, but this is the very first time I've ever seen a claim that the 14th Amendment itself is invalid because it was a "piece of legislation". Every single Constitutional Amendment began literally that exact same way.