The Alito dissent is fairly straightforward -- Congress has tried to fix the language of the bill several times, but has been unsuccessful, indicating that the legislators have not universally agreed that this is how it should be applied. They claim that fixing this should be Congress's job, not the court's, and the attempts to amend it are clear indicators that Congress recognizes that the law does not current protect those classes.
> Title VII of the Civil Rights Act of 1964 prohibits employment discrimination on any of five specified grounds: “race, color, religion, sex, [and] national origin.” 42 U. S. C. §2000e–2(a). Neither “sexual orientation” nor “gender identity” appears on that list. For the past 45 years, bills have been introduced in Congress to add “sexual orientation” to the list,1 and in recent years, bills have included “gender identity” as well. But to date, none has passed both Houses.
> Last year, the House of Representatives passed a bill that would amend Title VII by defining sex discrimination to include both “sexual orientation” and “gender identity,” H. R. 5, 116th Cong., 1st Sess. (2019), but the bill has stalled in the Senate. An alternative bill, H. R. 5331, 116th Cong., 1st Sess. (2019), would add similar prohibitions but contains provisions to protect religious liberty. This bill remains before a House Subcommittee.
Kavanaugh makes a very similar argument, even citing the same attempts to amend, and I'm not sure why he didn't join.
> The Court has previously stated, and I fully agree, that gay and lesbian Americans “cannot be treated as social outcasts or as inferior in dignity and worth.” ... But we are judges, not Members of Congress ... Our role is not to make or amend the law. As written, Title VII does not prohibit employment discrimination because of sexual orientation.