Prostitution for goods is also prostitution and illegal wherever prostitution is illegal. One exception to this are arranged marriages and marriages for money that, in my view, are a form of prostitution by barter (if not human traficking in the former case) but often legal. Pimping is more often illegal than prostitution.
Assuming you want to use Bitcoin to evade the law, then it is not the absolute best way to pay for prostitution either because it's not private (though it can be anonymous).
Sourceless but very googlable.
