If a young adult or an adult man or woman does not marry, it seems to be rather impossible to remain chaste and free of corruption. It is a difficult problem to find a young adult out of millions, not married, yet be chaste and sinless. If we find a youth who is truly chaste and not married, then we may say she/he is one of God's Saints. Avoiding commitment of sins, remaining immune from corruption, being safe from the outburst of the instincts, and not being married at the same time is something only the Prophet Joseph could do.
A house in which an unmarried man and a woman live is not safe from corruption. If the man has no wife and the woman no husband, and their sexual instincts are alive with the pressure of lust, then those two have various mental, family and social problems and live in corruption. Marriage is a natural and divine law. It makes some problems easier to solve which are concerned with keeping the youth chaste and pious.
The establishment of a household in society must be founded on a healthy and peaceful basis. The couple live together by marrying and respecting each other's rights. Wherever a Muslim household is established it must be based on God's revelation and in the remembrance of Him day and night.
(Lit is such a Light) In houses, which God Hath permitted to be raised to honor; for the celebration, In them, of His name: in them is He glorified in the mornings and In the evenings, (again and again), [Holy Quran: Nur 24:36]
In such a house, with such attributes, one finds a household of believers in which worshipping God flourishes. God has ordered a marriage to take place there and the couple obeys all divine, humane laws. The Glorious Quran orders that marriage should take place so that a man and a woman's problems are solved through the realization of this tradition and they - being the future teachers of some children - will remain incorruptible.
A man and his wife establish a mutual life. Being in harmony with each other they make a home for God's remembrance. In such a house, the couple are real servants of God and their offspring are the fruits of virtue. Their behavior and morality are signs of divine etiquette and the traditions of the Prophets. When a believing couple get married, they both feel responsible to follow the divine laws. They find each other to be a helper to the other. They are two loving friends, two intimate companions, two sources of faith and two pillars of love and kindness. Thus they protect life from difficulties. If a difficulty should arise, they solve it easily and confront it with the arms of patience and fortitude.
source : the Islamic Family Structure By Husayn Ansarian
