As I see it, where is the victim in adult prostitution? I think it should only be illegal if you are dealing with people being forced or tricked away from their parents and sold or used as sex slaves--that should definitely be highly illegal (and treated as rape and kidnapping).
However, paying for sex is no one else's business except the customer and the worker. If the person doing the sex is the one making the profit, no one is forced against their will to do it (if this person is under a reasonable age of consent, or if the customer is underage, they might make any parents or guardians handle such cases). But, so long as both parties are in a position to consent, no offense has taken place.
And this goes for drugs. I do not agree in any wisdom for anyone to start using drugs. But, that should be up to them. It's their brain and their liver--which using drugs can and does ruin. If someone is selling drugs, as long as the product is properly disclosed, no offense has taken place (however, if someone sells something and misrepresents it as drugs, or if they sell something that unknown to the customer contains drugs, you have a fraud case).