From https://www.wsj.com/articles/supreme-court-rules-for-gay-rig...:

> Title VII of the 1964 act broadly prohibits employers from discriminating against workers, including in hiring and firing decisions, based on an employee’s sex.

> The Trump administration argued that the 1964 law doesn’t cover LGBT workers.

There's a debate around whether "employee's sex" includes "employee's sexual orientation."