If porn messages include the names of neighbours, attractive or unattractive, then someone in the recipient household has been searching these websites and entering these names.
Nothing to do with wifi, passwords or any technological defect. Stop pretending the elephant isn't in the room. Sit every other adult person in the household down one by one and discuss the matter with them. One of them is responsible.
Why is this something you are afraid to discuss? If lots of porn sites suddenly started appearing in my inbox, or DH's, I would imediately mention it. Why wouldn't you?