On the topic of Oxytocin, it is true that it's associated with sex, but it's not only "released" after having sex. The fact is that having a good healthy relationship actually increases the chances of high secretion of Oxytocin when having sex and the lack of security within a relationship decreases the level of Oxytocin within the body. Hence you get the term "having emotionless sex"!! High levels of Oxytocin lead to a higher sense of bonding when having sex, so it's not the sex that leads to Oxytocin, but the Oxytocin would lead to a tense sexual experience and feeling of pleasure.
Linking that to the prostitution emotionless sex, that would explain why prostitutes and "customers" don't feel a bond when having sex, it's because they didn't have any kind of pleasurable memory or experience that built up the existence of Oxytocin to be released during sex.