How is Mutah different from prostitution (from a non-Muslim point of view)?

How is Mutah (temporary marriage) different from prostitution? This was a question asked by an atheist friend of mine and I couldn't answer him satisfactorily. **Edit:** The question is not about that its legitimate anymore but one would have to justify the circumstances (from a non Muslim point of view) in which it was once allowed. (From Sunni perspective)