I don't think you can equate prostitution with slavery. There are many things that are sick/disgusting but legal.
There are people with feet fetishes who pay people to let them rub their feet. Should those people be arrested? is that the same as slavery? Not even close.
If it doesn't hurt/effect you in any way, why should you care?
Prostitution is only illegal in countries controlled by religious populations (like the USA) or religious governments (like Iran).
You support the right for a woman to get an abortion because its her body, but despise the same woman for making the choice to sleep with a man for money. This is a huge double standard.
Tell me the difference between paying a man/woman to; wash your car, rub your feet with oil, or have sex with you.
All are services provided by someone who has a choice of what they do with their lives and are trying to make some money.
As far as i can tell, no one is getting hurt, no one is forcing anyone into anything, no one is stealing from anyone, so where is the crime?
Don't get me wrong. I wouldn't advocate for anyone to go into this line of work. It disgusts me as well and i couldn't imagine the appeal for either party. But there is no logical reason for it to be illegal.