They will certainly not like it. This is one of the reasons why ending prohibition is so hard. There are three groups with vested interests in keeping it around:
Driving down prices, making them affordable, and making it a more open market so to speak, sure it sounds good to the person who wants the freedom to do so, it's liberating to some, but what about those people who are cashing in on this multi billion dollar industry? The drugs lords, mafia, and the big criminals behind this. What happens to them? Are they just going to go away?
Presumption. Legalizing drugs will not increase demand for kiddie porn, and the black market cannot create demand for that product, only supply it.
Now lets think about what other criminal activity do we have underground that makes a profit? Terrorism, Porn, Kiddie porn, human trafficking, other violent crimes...are we ready to face an even bigger explosion in any of these areas and are we ready to face even more heinous criminal activity for profit?