When you engage in an act that can lead to life, you do have responsibility. This applies even if you did not desire pregnancy, or used contraception, because there are known failure rates of all methods.
And we see this reflected in current law. A man who has sex with a woman who becomes pregnant is required to pay child support, and often expenses for pregnancy, whether they wanted a child or not, because the man made the choice to engage in an activity that could lead to life and responsibility.
Which then means you do in fact recognize that responsibility can stem from the sexual act. You just want to limit it to avoid the implications.
No, in fact I have explained why. The notion that you owe a child a car, or trip, does not follow from engaging in an act that leads to life. You don't have to travel to Europe, or have a new car to have life.
And even in the case of the organ donation, you are drawing a line quite different than what I referenced in regards to the responsibility owed by the man and woman engaging in sex.
The person that engages in an act that can lead to life takes the chance that this will occur. This potential--life, is an inherent reality to the act.
And to argue that parents should feed the child they gave life to--but only if they don't kill it outright, is non-sense. If they owe food to sustain life, based on the responsibility of engaging in sex--and you indicate the man does, because of participation in the sexual act--then they certainly have no right to terminate the life they created, or else they are just dodging that responsibility which you already acknowledged they have, to care for the child.
And as noted, in some jurisdictions in the US where the man can be held responsible to pay for care of the woman and the new life, even while that new life is in the womb.
And you have introduced a false equivalency in the case of organ donation.
Supporting the new life in the womb is the natural act of that organ, and an understood process, and responsibility, that can come into play when you engage in sex. You have to use technology to overcome that natural progression--killing the life you have the responsibility to protect.
On the other hand, the notion that you would have to use technology to remove an organ to donate in the unforseen eventuality of a child needing it, is not not a forseeable duty when engaging in sex. They are not parallel.
One is the natural use of an organ, in a forseeable consequence of sex.
The other is the removal of an organ in an extreme eventuality that is not anticipated. And even then, the parent would have to be a compatible donor. It is a great act of love if the parent donates.
I am not assuming the conclusion. I am pointing out that those who say the man and woman have an obligation to feed a new life that they created, obviously should not try to circumvent that responsibility by actively killing it. They should take responsibility for their action, and support the new life.