Well, if you create a message board that accepts images, and you make it so there is no technical way to remove those images, you are implicitly including child porn in your definition of free speech.

It's typically included on lists because of the implications. One either takes the position (1) child pornography should be allowed to be shared, (2) child pornography should be suppressed, but may be shared, or (3) child pornography cannot be allowed to be shared.

(3) obviously requires very different technical solutions (like: no private encryption, ever) than (1).

I fail to see any relevance to a discussion centered around debating implications and boundaries of 'free speech.'

Do you think I should be allowed to possess encryption, through which I could share child pornography with others, without being detected?

That sounds suspiciously similar to the historical argument about pornography in general: no one could defend that; it's bizarre to bring up.