Sorry SC, you haven’t even made it off first base.
The Church does not teach this.
Papal infallibility does not cover my posts on this forum. However, I can only use the information available to quote the Church’s position on this. For example, from the Catechism of the Catholic Church:
On good and evil actions:
1756 It is therefore an error to judge the morality of human acts by considering only the intention that inspires them or the circumstances (environment, social pressure, duress or emergency, etc.) which supply their context. There are acts which,
in and of themselves, independently of circumstances and intentions, are always gravely illicit by reason of their object; such as blasphemy and perjury, murder and
adultery. One
may not do evil so that good may result from it.
On Divorce and Remarriage
2384 Divorce is a grave offense against the natural law. It claims to break the contract, to which the spouses freely consented, to live with each other till death. Divorce does injury to the covenant of salvation, of which sacramental marriage is the sign. Contracting a new union, even if it is recognized by civil law, adds to the gravity of the rupture:
the remarried spouse is then in a situation of public and permanent adultery:
“If a husband, separated from his wife, approaches another woman, he is an adulterer because he makes that woman commit adultery, and the woman who lives with him is an adulteress, because she has drawn another’s husband to herself”
Sex outside of marriage
2390 In a so-called free union, a man and a woman refuse to give juridical and public form to a liaison involving sexual intimacy.
The expression “free union” is fallacious: what can “union” mean when the partners make no commitment to one another, each exhibiting a lack of trust in the other, in himself, or in the future?
The expression covers a number of different situations: concubinage, rejection of marriage as such, or inability to make long-term commitments. All these situations offend against the dignity of marriage; they destroy the very idea of the family; they weaken the sense of fidelity. They are contrary to the moral law.
the sexual act must take place exclusively within marriage. Outside of marriage it always constitutes a grave sin and excludes one from sacramental communion.
Familiaris Consortio Chapter 84
However, the Church reaffirms her practice,
which is based upon Sacred Scripture, of
not admitting to Eucharistic Communion divorced persons who have remarried. They are unable to be admitted thereto from the fact that their
state and condition of life objectively contradict that union of love between Christ and the Church which is signified and effected by the Eucharist. Besides this, there is another special pastoral reason: if these people were admitted to the Eucharist,
the faithful would be led into error and confusion regarding the Church’s teaching about the indissolubility of marriage.
I would be happy to see references from you to some magisterial documentation which supports your idea that sexual activity outside of the bonds of marriage does not put one in a state of mortal sin.