Sex is a reward in a relationship... let's hope it's not the ONLY reward. There are other things like love, friendship, support buddy, fighting buddy, and your personal coach all rolled into one. If you don't have the added pluses, then you simply have a sexual transaction -- not much different than street 'professional.'
Sex is an important part of a relationship... but it's role is on the same level as many other integral parts.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen