I think I did post a framework that answers those questions… @jburns makes good points.
Your reference to animalism was right on the mark, totally relevant to adultery. God teaches that the consequences of sexual sin make us like animals (slave to instinct, insensibility to His word, ceasing of prayer or unheard prayer, unfit to lead men.) Our pride in thinking sex on our terms is perfectly good for us adds to the grievance.
When someone dating or cohabitating tells you they think they’re just doing the same thing as you and your wife, you have to use your pastoral gifts to either guide them towards a fully understood and realized marriage or get them to separate to avert the total destruction.
If someone had a prior sexual relationship you just have to work with what you have in front of you. The important thing is that God will use their sexual relations inside their (hopefully eventual) marriage for their good. If they don’t marry he will use their celibacy for their good (another area where God uses something mundane, that looks indistinguishable from their own practices to the world.)
If you’ll forgive quoting a pope, I like how Ratzinger lays it out:
The marriage of baptized persons is indissoluble. This is a clear and unambiguous directive of the faith of the Church of all centuries, a faith nourishing itself from the Scriptures. It is a categorical directive, that is not at the disposal of the Church, but is given to the Church to witness and to realize; it would be irresponsible to give the impression that anything on this point could be changed. The “yes” of marriage in the Church participates in that definitiveness that in the definitive decision of God for man at the same time has become visible as human possibility. It continues the “decisive decision” of God for man in the decisive decision of man for man. Marriage is one of those fundamental decisions of human existence that can only be made completely or not at all, precisely because therein man as a whole is involved, as his very self, unto that depth where he, touched by Christ, transformed, is taken into his “I” opened on the cross and open for us all. This is what is meant when we call marriage a “sacrament”.
Marriage does not remain in the “law”, but is incorporated into the Gospel as the reality of the decisive decision, and is structured by this decision: as Christian. But this means that two fundamental tendencies of modern thought prove to be just at this point incompatible with the Christian faith, or that marriage is exactly the point where fundamental anthropological decisions become concrete and have to be determined one way or the other. First, the reduction of being to consciousness, in which only that which is present in a man’s consciousness is what is truly valid for him (which practically means a throwback to the pre-Christian-Roman consent theory: if the consent ceases to exist, says this theory, then the marriage has ceased to exist. Theories such as these, that a marriage can be just dead and so no longer exist, are forms of this Phenomenologism, which reduces man to his consciousness, thus concealing from him just that depth that faith wants to open up to him.
Besides that – in much the same sense as what has just been said – is the reduction of being to time, which knows only the sequence of becoming and loses what is beyond it, the constancy of being. In contrast to this sale of man to “Chronos,” to the changing gods of the moment and to immediacy stands “Pistis” as fidelity, which in trusting [or in marrying: im Trauen] keeps man for the abiding and thus breaks open the circle of the recurring, gives man the possibility of growth, of going ahead, which has fidelity as its condition …
The Church is the Church of the New Covenant, but it lives in a world in which the “hardness of heart” (Mat 19:8) of the Old Covenant remains unchanged. It cannot stop preaching the faith of the New Covenant, but it must often enough begin its concrete life a bit below the threshold of the scriptural word. Thus it can in clear emergency situations allow limited exceptions in order to avoid worse things. Criteria of such action must be: an act “against what is written,” is limited in that it may not call into question the fundamental form, the form from which the Church lives. It is therefore bound to the character of exemption and of help in urgent need - as the transitional missionary situation was, but also the real emergency situation of the Church union.