I think that it would increase it, because contrary to what people seem to think here, prostitution is very low on people's ideal career choices. Increasing demand is going to increase the need labor, and I don't think anyone here is advising their sons or daughters to have sex with strangers for money. SO trafficking would increase.
If something is legal then workers have protection, can form unions and have rights. It's more difficult in a legal environment to have people who are coerced or forced to work.
If prostitution is illegal then it will be run by people who do not obey the law and will happily traffic people and have people people working illegally or forcibly.
Current laws seem to penalize the sex worker and the buyer, not the traffickers. Legalizing this would let both parties involved feel safer, no repercussions from calling the police if something goes awry.
