Question: Trying to figure out the fallacy here, I can't put my thumb on it.

It does sound like argument from ignorance (see Kahane, Logic and Contemporary Argument).

But there may also be a false dilemma. The argument assumes that only two choices are possible. (Getting into factual area here, many scientists have researched various intersex sates.) When someone tells you in a loud voice that there are only two choices, caution is advised.