Nope, To answer your question. I don't think Vote selling is like prostitution. Because If it is anything, It is Vote selling and nothing more. You are trusting another entity to cast your vote on behalf of you and you will get the share of profit or may not. 
because if you go by that logic the SP delegation is also prostitution and your savings account in the bank which generates interest also prostitution. 

You are just putting your assets to work. Because if you are able to cast the votes by yourself by curating good contents you can make some decent returns on your time spent, And since you are not able to do it by your self you are trusting the other parties to help with it. Now it is the responsibility of the entities to take care of this trust placed in them. 
Same way like the bank take cares of their assets.