Matt Zwolinski contemplates exploitation:
An exchange can be mutually beneficial and yet unfair or degrading. If you are drowning in a lake, and I row by on the only canoe in sight, it is morally wrong of me to make my rescue of you contingent upon your signing over the deed to your house. Granted, you would be better off taking my deal than passing it up.
We can grant that even exploitation of a mutually beneficial sort is a serious moral wrong, but it simply does not necessarily follow from this that it is something that governments ought to prohibit it. ... Perhaps prohibiting exploitative transactions will lead would-be exploiters to offer mutually beneficial deals on fairer terms. But, as is illustrated in the case of sweatshop labor, there is reason to worry that prohibiting exploitation by, say, mandating safety improvements or a higher minimum wage, will make the package sufficiently unattractive to would-be exploiters that they wind up prefering to make no offer at all.
Hence the case for legal prostitution.