The radical separation of the two ends of conjugal union entails two consequences. First of all, it imperils the very existence of the family cell, notably in favoring free love before marriage. Then, bit by bit, it leads to a state of mind that rejects life and is even haunted by death. Since procreation is an evil to be avoided at any price, inevitably one must put to death the one who becomes an obstacle to the sole good which spouses seek in the conjugal act, namely, carnal union with the pleasure linked to it.
VADEMECUM FOR CONFESSORS CONCERNING SOME ASPECTS OF THE MORALITY OF CONJUGAL LIFE
http://www.vatican.va/roman_curia/pontifical_councils/family/documents/rc_pc_family_doc_12021997_vademecum_en.html
CBCP PASTORAL LETTER: "LOVE IS LIFE" (1990)
http://www.cbcp.net/documents/lovelife_90.html