Functionalist approach to any forms of deviance (including prostitution if it is defined as deviant):
It is functional for society because:
1. In defining the various forms of sexuality which are deviant in society we are confirming the boundaries of acceptable social behaviour.
2. In turn this reinforces the sense of certainty in society..this is especially important in modern societies which are based on social change rather than 'traditionalism'
3. Nevertheless deviance also challenges those boundaries and the controversies about whether any specific act should be labelled deviance allows for controlled levels of social change
thus deviances is functional in providing social boundaries, reinforcing social norms, providing levels of certainty, preventing anomie (the form of psychological uncertainty we suffer if social rules are unclear, yet permitting socially controlled levels of social change.