The sexual union of man and woman begins a marriage. To perfect that marriage, both the man and the woman must purify and perfect their love, by eliminating the defects (lust, pride, envy), giving birth to the perfections (love, chastity, altruism), and serving others selflessly.
In other words, nature is blind to beliefs, morals, theories, cultural traditions, or ideas about sex; nature is chemistry, biology, action and consequence. If one has sex, one creates a union of souls. If one has many partners, one is an adulterer. If one has the orgasm, one is a fornicator. All adulterers suffer the same consequences; this is proven in observation of facts. All fornicators suffer the same consequences; this is proven in observation of facts. These are not beliefs or "gnostic" ideas, these are actions and consequences in your chemistry, biology, and psychology.
Marriage is the same: it is not a belief or idea, it is a fact of nature.
“Nothing is easier than self-deceit. For what each man wishes, that he also believes.” —Demosthenes
"Do not worry; cultivate the habit of being happy." —Samael Aun Weor