There is clear precedent for YC taking a political stand by uninviting investors. Only five years ago, Paul Graham was willing to uninvite investors from Demo Day based on their support for the Stop Online Piracy Act (SOPA). https://techcrunch.com/2011/12/22/paul-graham-sopa-supportin... Paul asked, "If these companies are so clueless about technology that they think SOPA is a good idea, how could they be good invest…
I disagree that opposing SOPA is just political, supporting SOPA threatens innovation which threatens the startup community which in turns threatens YC's bottom line. Therefore opposing SOPA and denying investors access based on their support is a business decision.
Silicon Valley already has tremendous diversity issues and a reputation as out-of-touch. If the future of America and the world only gets more diverse, which seems to be the case, then YC has a vested interest in being on the right side of history.