A key clause not included in the headline and only briefly mentioned in the article is that this doesn't ban all step-porn. It only bans the step-porn if one of the actors pretends to be under 18.

From gov.uk:

The first of these vital measures will ban anyone from possessing or publishing harmful pornography that shows incest between family members, and sex between step or foster relations where one person is pretending to be under 18.

A further amendment will criminalise the publication and possession of pornography where an adult is roleplaying as a child.  

(edited to update link)