Procreation Recreation Co-creation
Until the last century, at least in the west, procreation was the accepted norm of sex in a relationship. It's only been in the last 50 or 60 years that recreation began to gain some acceptance, and only now is the evolutionary meaning of sexual relationship as co-creation starting to gain acceptance.
While they can obviously overlap one another, each of these has a different meaning and purpose. and each carries it's own definition of what constitutes a successful relationship.