It is not true that the only morally copulation is one for the purpose of procreation.
God commanded that a man and woman be married (leave his father and mother and stick to his wife, and the two will be one flesh). This arrangement provides the tremendous love that can exist and abide between a man and a woman, plus it provides a secure home for the raising of children.
People can get married and not have children, which sometimes happens, and marriage can be happy anyway. Sometimes the children are grown and have their own families, and there can still be a marvelous relationship between the married parents. People can also lose their spouse after many years, and marry another happily. I have seen all of these happen, and it shows what an awesome gift from God romantic love and marriage between a man and woman is.
The Song of Solomon references romantic love and loyalty between espoused couples and married people. First Corinthians chapters 7 and 13 also describe marital love, and agape (higher, principled) love.