Fornication is “carnal union between an unmarried man and an 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”. The Catechism teaches that the sexual act “must take place exclusively within marriage”, and that outside it “it always constitutes a grave sin and excludes one from sacramental communion”. An intention to marry later does not change this, since “carnal union is morally legitimate only when a definitive community of life between a man and woman has been established”.
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