I agree, many jobs should be made illegal. It's not just prostitution.
I do not think, for example, it should be legal for a woman to be paid to have babies for someone else - but it's legal in the US and I am currently arguing with a mate of mine who thinks it's 'just nine months sitting around'. 
I don't think workers should be forced to be exposed to carcinogenic pesticides without consent (or with, really).
I don't think workers should be sold into slavery as children, then conned into believing they have to stay to keep their families safe.
There are many jobs that should be illegal. No one campaigning against prostitution thinks these other things are ok, so I don't know why they're being brought up.