Supreme Court Considers Efforts To Redefine Sex To Mean/Include Sexual Orientation and "Gender Identity" (?)

Not exactly sure what's going on here.
You can't redefine sex to include sexual orientation and "gender identity"... That's just idiotic. Surely the Supremes aren't going to let that kind of nonsense pass...right?
However, I've never quite understood why one couldn't argue something like the following:

(a) Discriminating against Smith because he is homosexual is a type of discrimination based on sex; the reason is that Smith is being discriminated against for having sex with men, whereas women who have sex with men face no such discrimination.

(b) Discriminating against Smith because he wants to wear dresses etc. is a type of discrimination based on sex; the reason is that women are permitted to wear dresses etc.

Perhaps the response is: to discriminate against someone on the basis of their sex is to discriminate against them on the basis of their sex independently of any behaviors they engage in... Whether or not that's true should be a simple legal question, I'd think..
   But, anyway: I could see this going either way so long as the arguments deployed on the one side are similar to (a) and (b)...but I can't see any even vaguely rational way to argue that sex includes sexual orientation or "gender identity" (the latter not even being an actual thing).
   This is the kind of reason I now want a conservative court. I don't trust liberals to not do something insane here...
[Of course even if (b) is right, one might argue that I shouldn't have to hire Smith if he actually believes that he is a woman, anymore than I should have to hire him if he thinks he is Napoleon. Wearing dresses is one thing--in that case, Smith merely defies clothing conventions. Believing that he's a woman is a different thing entirely.]