I'll mention seatbelt laws again. The government already has all the power it needs to make porn and prostitution illegal.
No first amendment issues are raised when it comes to seatbelt use. Porn, on the other hand, is a type of film-making (or photography, or writing, whatever). A legitimate argument can be made that it is a type of speech. It does put forth a message- i.e., that promiscuity is a good thing.
The government already has all the power it needs to make porn and prostitution illegal.
Prostitution, yes, since that isn't a type of speech. Porn falls under protected speech.