Marijuana is from a family of plant many people are allergic to. I'd be happy for it to be legalized as long as where you smoke it is subject to a similar level of conditions as cigarettes.
Prostitution it depends a lot on the regulations, if its done in a smart way its far better, if its done haphazardly then its a huge issue.
Pornography can be tricky as to where to draw the line. Obviously child porn should be illegal, straightforward vanilla porn should be legal. Most BDSM and fetish stuff should probably be legal, but where does one draw the lines on say animals, exhibitionism, heavy SM or torture, fake rape, etc.
The "sin tax" represents the fact that its a heavy regulated industry, and a luxury rather than a necessary good. This means the government can afford to tax it heavily to pay for regulation and essential services. I also think legal marijuana with 30% taxation would be far cheaper than illegal marijuana. In fact flooding the market to that extent should make it cheaper than medical marijuana is currently.
I think people will be happy that they will be able to get these things without fear of arrest, and if the way to make them legal is through government regulation then charging for that regulation is necessary.