I think illegal prostitution does make it less safe if those laws are actively enforced for 3 reasons, 1/ for the client if its illegal they would have to go looking for it in some dodgy places. 2/ for the prostitute, if they have to fear prosecution, crimes committed against them may and probably do go unreported. And 3, if its illegal and unregulated both client and prostitute might not be clean and regularly checked for sexually transmitted diseases.