We feel your question may be an over-generalization since many married couples do not engage in sexual activities outside the bonds of marriage and many unmarried couples do engage in fornication. The usual reasons for fornication in either case stem from selfish desires. Married couples may get bored with their spouses, fall out of love, etc. and seek change or excitement outside of marriage. Singles, on the other hand, would rather “test the waters” or “play the field” before taking the plunge into marriage. Both groups violate God’s righteous laws that forbid sexual relations outside of marriage. Why do some individuals not practice fornication, whether married or single? We believe that they put God’s word first in their lives, not their own personal desires or perceived needs. They value God’s institution of marriage as sacred. The vows to love, cherish and be faithful for better or worse, in sickness and health unto death are serious promises to Christians who choose to live in accordance with God's will. In the sight of God the marriage covenant requires wholehearted commitment, exclusivity, and unswerving loyalty to one's partner. Jesus adds in Matthew 19:6, “So that they are no more two, but one flesh. What therefore God hath joined together, let not man put asunder.” Since the Christian marriage is also an earthly picture that symbolizes the heavenly union of Christ and his bride, when marital problems arise it is the obligation of both parties to prayerfully ask the Lord for guidance to find a way to set matters right. When a person strays and engages in fornication outside the marriage covenant, he or she is not respecting God’s sacred laws and is looking for a means of escape.
A marriage must be nurtured and requires time and work. Married couples that honor God’s word are willing to take the time and do the work. The Apostle Paul gives much good advice to husbands and wives in Ephesians 5:22-33. He ends by saying, “Each of you also must love his wife as he loves himself, and the wife must respect her husband.” (Ephesians 5:33)
You wanted to know why people who are blessed with spouses engage in fornication and those that are not married do not engage in fornication. The answer is the same for both classes of people. Anyone who honors God’s laws and is willing to sacrifice his or her own desires to do God’s will, will not engage in fornication, whether married or single.