After a day of thinking about it, here's what I'm left with...
- The male headship principle regarding spiritual leadership in general is well supported by Scripture. And in particular, the fact that all the apostles were men and apostolic authority is linked to the New Testament books gives support to the idea that God would use men only to write Scripture. So we can strongly expect that all the Bible authors would be men on that basis. It fits what we know about how God works. Still, I'm mindful that God has at times worked through people we'd least expect.
- What we know about what was culturally acceptable at the time also suggests the authors would all be men. But that's an argument from history and culture rather than one based on God's character, so I'm less interested in it.
- 2 Peter 1:21 specifically mentions men: "For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit." But might it be possible that Peter is using the word in general terms to mean "human"? Especially given the main point of the verse, that sounds like a reasonable possibility.
So I'm left with a strong likelihood that all the Bible authors were men, but something short of certainty. I like the argument from the male headship principle best. And I'd like to be able to say with certainty that the authors were men because that would add support to the male headship principle, which I like to defend. But to do so is a circular argument ("All the authors were men because of male headship. And male headship is evidenced by the authors all being men").
Am I missing something that settles the question with more certainty?