There are many people leading a single (unmarried) life for many reasons. Christian Priests, Religious and Nuns of some denominations forsake the worldly life with some spiritual motives and remain as singles practicing chastity. Some hermits and swamis also renounce married life for attaining spiritual ecstasy.
For normal people who want to lead a family life should seek and find a suitable life-partner and get married. Such people are only entitled to have sex with the partner. Anything out of it is non-ethical and unacceptable...!