Non-Sequitur: If you stop and think about it for a moment you would realize how ridiculous that statement is. In the first place, the Supreme Court was not enacting legislation. It was ruling on the Constitutionality of the act of secession passed by the Texas legislature. A quick reading of the court's decision would have uncovered that particular fact. In the second place, by your definintion then every ruling by the Supreme Court, or any other appeals court for that matter, is ex post facto since every decision is a ruling on something that has already happened. That's the way the system works, especially the Supreme Court which cannot legally issue any sort of advisory ruling. So it's back to the drawing board for you I'm afraid.
I've read the Texas decision, longer ago than I care to remember (birthday alert for tomorrow) and know what it was about. The problem with what you are stating is that the Supreme Court was in effect enacting legislation because there was no law or Constitutional language regarding withdrawal in the first place. The justices were creating a prohibition out of whole cloth.
When the Supreme Court rules in cases, it is ruling on acts that have already happened, but interpreting laws and/or rights that are already on the books (i.e. the Miranda decision had to do with the Fourth Amendment; Gore's attempted theft of the presidency was stopped upon the 7-2 ruling that the ever-shifting vote standards violated the Fourteenth Amendment). The Supreme Court does not have the authority to enact legislation or amend the Constitution which is what it did in the Texas ruling.
Let's try this again. The Supreme Court does not enact legislation. It cannot. It has no power to do so under the Constitution. But it does have jurisdiction to rule on the constitutionality of legislation passed by the states and to determine if such legislation violates the Constitution. And in the case of Texas v. White the court determined that the Texas acts of secession were in violation of the Constitution. The fact that you do not agree with their interpretation means nothing. They don't need your approval for the ruling to be valid. The fact that you believe that the court was, in effect, making legislation means nothing. The court did not make legislation, the Constitution does not grant them that power. The court interprets the Constituion as it relates to the case before them, and it this case they made an interpretation that you don't agree with. But your agreement is not needed for the decision to be valid. And Texas v. White was a valid decision and will remain so until the Constitution is amended or the decision is modified by a future court. So you are welcome to your opinion. I'm not going to try and talk you out of it. But the fact that you believe the court made legislation and that you believe that there is nothing in the Constitution preventing unilateral secession is meaningless from a legal standpoint.