Prostitution is being paid to have sex with someone. What is difficult to understand about that.
The question to ask the participants is, would (the girls) be prepared to sleep with this man if he wasn't paying them, in other words, would they be happy to be in a committed relationship with him instead of him paying them for sex? For the men, would they be prepared to have a proper committed relationship with the girl as an alternative to paying her?
I am pretty sure I know what the answers would be - and that makes what they are doing prostitution.