If you don't know whether the person you're paying needs the money to survive, it's immoral to buy "sex." You don't know to what degree it's "sex" or they are forced into it, which is called "rape." Banning the purchase (the "Nordic model") is the same as banning other immoral acts.

It's the same with porn by the way. You don't know if the women you're watching are on drugs, being threatened by men, or just needs the money to survive. So what what are you watching? "Sex" or rape?