> If there are indeed biological differences between sexes that could help explain different aptitudes/predispositions for different intellectual pursuits, should it be firable to say so?
First, I'll note that "should it be firable" in the question is a little bit ambiguous. I'm interpreting it as "should a company be allowed to fire an employee for this", and not as "should a company fire an employee over this".
If you believe in at-will employment, then yes. Truth does not protect you from being fired.
I'm not a huge fan of at-will employment in general, but I think even in this context the 1A freedom of association makes it problematic to prevent companies from firing people for their speech. Even truthful speech.
As an individual has the right to speak, without interference from government actors, the employer has the broad right to choose who they associate with. Restricting that right is problematic.
> 2. If boys and men are discriminated against in order to pursue equal outcomes, should they be allowed to voice opposition?
Of course they should be allowed to voice opposition. The government should not interfere with their speech in any way. Further, the government should not discriminate against those individuals based on that speech.
However, as they are allowed to voice their opposition their may be social consequences for doing so. Speech often carries social consequences—some I agree with and some I don't. For me, how appropriate or proportional those social consequences are depend very significantly on the content and tone of the particular voiced opposition and the social response. That makes it somewhat harder to respond to a general question.
You are free to challenge my responses. I'm happy to discuss these ideas.
Some similarly constructed questions I would return:
1. If there are not biological differences between sexes that could help explain different aptitudes/predispositions for different intellectual pursuits—but a coworker asserts that such differences exist, that your sex is the one with less aptitude—could you see how upset employees and applicants of that sex?
2. If women have traditionally been discriminated against in this industry, and a co-worker voices support for this ongoing discrimination in a way that makes them uncomfortable, should they be allowed to complain about that co-worker? Should their complaints be taken seriously?
I similarly won't challenge you if you answer.