Men who pay for sex cant get it for free and aren't willing to take it by force.
Secondly women use sex for their advantage. Sex is their only means of manipulation for men. They know its something men desire badly so they exploit it by asking for money
It's not like that men who dont have girl friends are paying for sex. Many wants to spend nights with different type of girls every day, many prostitution centers nowadays offer australian, chinese, arabic, etc, etc type of girls in one place. Dont use the same brand daily, have a change, is also a main reason.