Child porn is unprotected speech that does not enjoy first amendment protections. What you are advocating for is giving the government control over forms of protected speech, which is a whole different ball of wax. Creating precedent that the government should regulate acknowledged protected speech is not a good idea at all.

> What would be novel in twitter's case, is that rather than recognizing that the phone company is a natural physical monopoly and hence must not refuse service to the politically unpopular, twitter is more of a natural social monopoly because of network effects

Twitter is not a monopoly.