The spouses’ union achieves the twofold end of marriage: the good of the spouses themselves and the transmission of life. These two meanings or values of marriage cannot be separated without altering the couple’s spiritual life and compromising the goods of marriage and the future of the family.
The conjugal love of man and woman thus stands under the twofold obligation of fidelity and fecundity.
"By safeguarding both these essential aspects, the unitive and the procreative, the conjugal act preserves in its fullness the sense of true mutual love and its orientation toward man’s exalted vocation to parenthood."157
Sexual unity within marriage must be both unitive and procreative. Procreative means that it must be done in a way that could achieve life. Fertility (the ability to conceive) has no impediment on achieving that twofold end.
The Gospel shows that physical sterility is not an absolute evil. Spouses who still suffer from infertility after exhausting legitimate medical procedures should unite themselves with the Lord’s Cross, the source of all spiritual fecundity. They can give expression to their generosity by adopting abandoned children or performing demanding services for others.