Slippery Slope. Once you begin down it, the devastating consequences multiply quickly.
In a society based on virtuous values and an understood moral code, a man and a woman get married, and they have children. The kids call their parents father and mother, or mom and dad. The parents teach the children their values, and the kids carry out what they learned when they become adults, and the cycle continues and continues as long as the society wishes to remain within those virtuous boundaries.
Along the way there are always exceptions. Some parents are unable to have children, so they adopt, or remain childless. Some men or women decide not to marry for a variety of reasons, be it career, or a desire to keep their lives completely to themselves, or a myriad of other possibilities. The exceptions may be outside the norm, and represent a small minority of the population, but nobody really worries about them.