"Fornication is a carnal union between an unmarried man and unmarried woman. It is gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses and the generation and education of children." CCC 2353
Social science studies reveal that cohabitation is a preparation for divorce. Cohabitation sets up a mind set that if the relationship gets tough the couple dissolves the relationship without going to court for a divorce. Not engaging in sexual intimacy before marriage is a preparation for marriage since abstinence requires commitment, self-control, and perseverance – all essential virtues for a successful marriage.