The scope of federal non-discrimination law, just as with state non-discrimination law, is a question for the legislature. In recent years, bills have been introduced annually to expand Title VII to include sexual orientation as well as sex. None have passed, which is prima facie evidence that this is a political issue that is no business of Supreme Court. This should be a slam dunk for the Court, especially if Chief Justice Roberts cares, as he says he does, about the Court's integrity and reputation.
This has nothing to do with the wisdom of extending non-discrimination protection to gays and cross-dressers (the former I strongly support). It has to do with who makes these decisions - our elected representatives or unelected judges. Obviously, we know the answer to that question - but the left is in the business these days of intimidating judges to impose their agenda when the people fail to cooperate, so who knows.