SCOTUS does not consider or rule on hypotheticals, only the specifics of a given case. And generally, judges will rule only based on the individual attorneys' arguments, not other arguments they can think of but the attorneys did not make.

I don't see what that has to do with anything. Saying that "congress should change the law because the law is vague" isn't a ruling. The Supreme Court has weighed in on far more vague and unclear laws before. It's basically just saying that they don't want to touch the issue.

That's not the argument the dissenters are making. They're saying that the fact Congress has tried several times and failed to add specific orientation and transgender provisions is evidence they shouldn't be included in "sex" (otherwise Congress would see no reason to add those specific provisions). Just because we happen to disagree with the argument doesn't necessarily make it a bad legal argument.