The positive intentional act, I guess, is disseminating the material. Proving intent and various ancillary concepts like knowledge, etc, are the bread and butter of prosecutors, so I don't think the act would be as difficult to enforce as you suggest. Sure, there might be corner cases where you can say "oops I forgot to password protect that Dropbox link" but certainly porn websites that don't do age verification will have a hard time arguing their conduct was not intentional.