The principle of double effect does not apply when a woman knowingly chooses both to take an abortifacient contraceptive for a medical purpose (first font/intention) and to have sexual relations (with the possibility of conception) because the chosen act has two evil moral objects in the second font: depriving the marital act of the procreative meaning, and depriving an innocent prenatal of life. Intrinsically evil acts are never justified by the principle of double effect, nor are they ever justified by intention (purpose) or circumstances. The first font of intention never justified an evil moral object. To say otherwise contradicts the teaching of Veritatis Splendor on intrinsic evil.
The death of an innocent prenatal, caused by deliberately taking an abortifacient contraceptive and then deliberately engaging in sexual relations, is the morally-direct result of intentionally chosen acts. This death is a direct abortion, not an indirect abortion. The woman who takes a pill for medical purposes is not obliged to engage in sexual relations. If she chooses to do so knowing that the act is contracepted, and that the act might result in an abortion, she is committing an objective mortal sin.
This is nice. Most of it is true, but it doesn’t pertain at all to the discussion because the object of the act in question is taking medication, not contraception.
It would be the same thing if you posted all about the Church teaching on murder as intrinsically evil on a thread about killing in self-defense. The objects are totally different because they are altered by a fundamental conditio which has become a “principal condition of the object.” Self-defense is not justified murder (for nothing can justify murder). It is simply a totally different moral act and object. In the same way, taking medication is not justified contraception (nothing can justify contraception). It is a totally different act.
Here are some other common acts that may seem at first glance like “justified” (as if such a thing could happen) intrinsically evil acts, but in fact they are utterly separate and distinct acts by virtue of having radically different (while somewhat similar in the physical) objects. All of these acts can have at times, evil effects in which case the traditional Catholic principles of double effect must be applied. The examples…
Killing an innocent person - Killing an unjust aggressor
Taking that which one has no right to take - Taking that which one has a right to take
Sexual intercourse with a non-spouse - Sexual intercourse with spouse
Removal of a healthy uterus - Removal of a cancerous uterus
Taking medication to impede fertility - Taking medication to cure a legitimate physical ailment
These acts as physical acts may look remarkably similar, but here a circumstance actually becomes a principal condition of the object itself. They no longer remain as circumstances to be weighed (as much as liberal proportionalist theologians would love that!). They become a part of the object itself. This is why we do not say that marital intercourse and adultery share the same object. They are radically different objects (one good, one evil) through the very important principal conditions that exist within each object.
I hope this helps you a bit to understand the theological underpinnings in this complex but deeply valuable moral teaching.