Catholics believe that the primary end of sex and marriage is procreation, albeit not ruling out secondary ends. That’s beside the point. People being an end in themselves means that before considering any outcomes of their lives their existence is good. E.g. if I have a kid who has Downs Syndrome their life is a fundamental good even though their outcome will probably be considerably worse than a healthy child. Many non-Catholics share this view. That does have implications for family size as it moves one’s attitude towards responsible generosity to having children rather than primarily considering the child a threat to or means to some other end.