The EEOC has ruled that “discrimination on the basis of sexual orientation is necessarily an allegation of sex discrimination,” based on the Civil Rights Act of 1964 which banned employment discrimination based on sex. The ruling will affect federal employees’ claims and all complaints filed with the EEOC, but it doesn’t completely ban the discrimination nationally. However, this ruling could pave the way for a federal court ruling to increase the sexual orientation discrimination ban from the current 22 states to all 50.
See "U.S. Agency Rules for Gays in Workplace Discrimination", Noam Scheiber, July 20, 2015