Sex is an important part of a relationship... so how would you know if you were compatible with someone enough to marry if you've never had sex with them?
Albeit, sex is not the most important thing... but without it, it tends to wear heavily on the relationship... even being one of the causes of its demise.
"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