Why is complete consent listed as a requirement for an act to be a mortal sin?

Why is complete consent listed as a requirement for an act to be a mortal sin?

Complete consent is a requirement for an serious evil act to be a mortal sin because it engages one’s free will. See the "Three Stages of Sin" according to St. Augustine.

So what kind of Sins are Mortal?

In order for a sin to be mortal, it must meet three conditions:

• Mortal sin is a sin of grave matter
• Mortal sin is committed with full knowledge of the sinner
• Mortal sin is committed with deliberate consent of the sinner

This means that mortal sins cannot be done "accidentally." A person who commits a mortal sin is one who knows that their sin is wrong, but still deliberately commits the sin anyway. This means that mortal sins are "premeditated" by the sinner and thus are truly a rejection of God’s law and love.

However the question seems to hint to the possibility that fear could possible diminish the culpability of a mortal sin.

Fear could possible diminish the culpability of a mortal sin, as the Catechism of the Catholic Church states:

1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offence, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.

Thus it remains true that full consent of the will is necessary for a grievous act to be a mortal sin, but fear may if fact mitigate one’s culpability.

Such sins of abortion must nevertheless be confessed to a priest. One is always free to explain all circumstances that lead up to the sin committed. God’s mercy is open to all!

The objection is irrelevant.

A decision on a matter that serious does not happen in isolation.

CCC 1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."

Put in the vernacular: so ya knew it was wrong and ya did it anyway.

As to complete consent.

1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice.

This answer assumes that the person in question is a practicing Catholic in full communion with the Church at the time of this choice. Any other case is irrelevant to the technical question you have on the matter of consent of the will.

A brief summary from the Catechism.

1874 To choose deliberately - that is, both knowing it and willing it something gravely contrary to the divine law and to the ultimate end of man is to commit a mortal sin. This destroys in us the charity without which eternal beatitude is impossible. Unrepented, it brings eternal death.

The choice for an abortion that you provide in your example was not the only choice available. Any assertion of coercion or fear as being sufficient grounds to mitigate the act of the will is unfounded (at least in part if not in full) since the doctrine of free will presumes agency on the part of the sinner. Beyond that, as a member of the faith community, there is available the counsel of the pastor and assistance from the rest of the faith community. Choosing not to seek pastoral counselling is hardly a mortal sin, but it's certainly an error in judgment. (Errors in judgment provide fertile ground for sin. Don't I know it.)

As to imputability

(Ref: Catholic Encyclopedia)

When the cause produces a twofold effect, one of which is directly willed, the other indirectly, the effect which follows indirectly is morally imputable to the sinner when these three conditions are verified:
first, the sinner must foresee at least confusedly the evil effects which follow on the cause he places;
second, he must be able to refrain from placing the cause;
third, he must be under the obligation of preventing the evil effect.

He's on the hook, as is she.

Beyond that, Jesus taught us: Be not afraid.

About the role of the will in Mortal Sin

From St Augustine: (Reply to Faustus XXII.27)

... something said, done or desired contrary to the eternal law, or a thought, word, or deed contrary to the eternal law. This is a definition of sin as it is a voluntary act. (Catholic Encyclopedia)

Lastly: the condition of being in mortal sin is not permanent.

The sacrament of confession and reconciliation is in place to return the pennitant into communion with the Church. The help's there, take advantage of it.

The edge case ~ suicide

In recent years, the Church seems to have adjusted its hard line on the suicide as a mortal sin.

2282 If suicide is committed with the intention of setting an example, especially to the young, it also takes on the gravity of scandal. Voluntary co-operation in suicide is contrary to the moral law. Grave psychological disturbances, anguish, or grave fear of hardship, suffering, or torture can diminish the responsibility of the one committing suicide.

It's not a get out of jail free card, but it opens the possibility of mitigation, or a conclusion that the consent of the will was significantly impaired. With that said, the Church then leaves the fate of the suicide in God's hands:

2283 We should not despair of the eternal salvation of persons who have taken their own lives. By ways known to him alone, God can provide the opportunity for salutary repentance. the Church prays for persons who have taken their own lives.

The Church will not condone suicide, but it need not condemn the suicide as a reflex. This leaves open the provision that, in the case of suicide, a defect of the will may overwrite "complete consent" or even sufficient consent. (per CC 1859)