Nope. It's not legal. What makes the porn industry legal is not the fact that someone else is paying. What makes the porn industry legal is that both parties are being paid to do it, or otherwise are doing it for commercial purposes rather than the fun of it. So long as one of the parties is fucking because he or she wants to fuck, it's prostitution if the other party is being paid.

In a sense, it's like professional fighting. It's lawful for two guys to beat the hell out of each other for money, but if only one is getting paid it's gangsterism.