A reason not to view prostitution as simply a matter of personal choice involving adults is that

Prostitution is legal in some countries, yet illegal in other countries. It can be viewed not as a matter of personal choice if the situation is seen in the perspective of the law, or rather, is seen in a legal perspective. If prostitution is illegal, then personal choice should not be held as an answer for choosing prostitution. The law says that it is illegal, and therefore, personal choice does not legalize prostitution in any way.