Sex is the participation of the flesh in the exchange of Divine Love; the most central part of the Sacrament of Matrimony. It is the taking up of the flesh into the exchange that already exists in the soul, with this taking up existing for the sake of procreation, that the children of God might be "born not of blood, nor of the will of the flesh, nor of the will of man, but of God" (Jn 1:13); created as He wills, not as the parents will, for "He made us, and not we ourselves." (Ps 99:3)
Yet, the world attacks God and lies about sex by turning it in towards itself and having it exist for its own sake for the pleasure of the participants. At that point, all manner of sinful acts become permissible because the measuring stick of participation in Divine Love has been replaced by the measuring stick of pleasure and vain happiness.
This is an all or nothing affair.