Carl Nassib of the Las Vegas Raiders is only the 16th known homosexual or bisexual player in the 101-year history of the National Football League (out of more than 26,600 players). He is the first active player to do so.
There have been hundreds of others, of course, but we don't know who they were (or are) because of the pressure to conform and the intense stigma of not being heterosexual. As Slate's David Kamp writes:
The NFL remains the most hierarchical and militaristic of all the major U.S. sports leagues.