Respect for life is not just mainly the consideration of the actual life that there is but also the potential or future life that is the normal result of the sexual contact of male and female species. So in the case of man, God's law and the Church's law ask that no obstacles be placed for the normal result of such union of male and female sex which is - human life. That is why the Church proclaims of purity, decency, and morality respecting marriage, family and life. Ultimately this boils down to protection of life.
What will happen if society gives free rein to all contraceptives and abortifacients that prevent the contact of male sperm and female ovum? None will be born anew. Life will be extinct. Maybe, yes, there will be rampant sexual licentiousness or other human abnormalities and enjoyments but these immoralities only invite God's inexorable chastisement as happened in Sodom and Gomorrah.
The reason for limiting sexual union only within legitimate marriage is likewise for the protection of life, stability of the family, the rearing and education of children which are the natural consequence of marital union. The Church's laws, following divine tradition, considers immoral the sexual relations outside of marriage. Why? It is likewise connected to human life, which is sacred. The fifth commandment (thou shall not kill) is closely linked to the 6th commandment (thou shall not commit adultery) (cf. Exodus chapter 20).