They claim it's settled science that sex is not binary, and it's hateful to discuss otherwise.
Quoting:
The first ethical principle in AAA’s Principles of Professional Responsibility is to “Do no harm.” The session was rejected because it relied on assumptions that run contrary to the settled science in our discipline, framed in ways that do harm to vulnerable members of our community. It commits one of the cardinal sins of scholarship—it assumes the truth of the proposition that it sets out to prove, namely, that sex and gender are simplistically binary, and that this is a fact with meaningful implications for the discipline.