> I am not a lawyer but this sounds to me like the Supreme Court just admitted that while the text of the law doesn't include protection for gay or transgendered persons, they are going to retroactively "understand the term" as though it does.

The rest of the argument makes it very clear why the conservative textualist justice who wrote the opinion considers that discrimination against homosexual or transgendered people is based in part upon the biological distinction between male and female: the assumption that the sexual preferences or identity of the individuals in these cases would be entirely unobjectionable to the employer if their biological sex was different. They're very clear on this and go on for several pages about it.

You may or may not agree with this argument or its applicability to these specific cases, but cherry picking something from the beginning of their actual argument and substituting a completely different 'retroactively "understand the term"' argument of your own invention is disingenuous at best