'Sexual orientation' was included for the first time in non-discrimination language of the 2011 collective bargaining agreement between the NFL and the NFL Players Association. NFL spokesperson Greg Aiello told Outsports it's part of the increasingly inclusive NFL:
It was introduced early on in the drafting process of the CBA by our lawyers as something that needed to be included. It parallels the policies and practices of our own office, which include non-discrimination, domestic partner benefits, and the hosting of PFLAG and related events.
The NFL itself has for years been inclusive in its front office. Goodell's predecessor as commissioner, Paul Tagliabue, has a gay son who works at PFLAG. It was Tagliabue who opened up the NFL to discussions about sexual orientation, and Goodell has obviously carried on that growth.
The CBA language, in case you missed it:
Section 1. No Discrimination: There will be no discrimination in any form against any player by the Management Council, any Club or by the NFLPA because of race, religion, national origin, sexual orientation, or activity or lack of activity on behalf of the NFLPA.