From Subbudh Parekh:
It is said that there are four main reasons why we engage in anything related to sex
– practical (procreation)
– pleasure (getting off)
– emotional (pair bond, ‘love’)
– spiritual (‘alchemy’, ‘self-knowledge’ etc)
This is an important question. I addressed it in the FAQ on this page, but the unmatched Bill Shor has created an excellent thread for it on the Hub.