Basically, I have previously presented this argument in favor of giving people a unilateral opt-out from paying child support:

https://www.reddit.com/r/Egalitarianism/comments/3xieil/an_argument_in_favor_of_giving_malebodied_people/

Anyway, as of right now, I still stand by this argument of mine. However, what I am curious about is this:

Is there a meaningful difference between causing someone to exist in the first place and between extending some poor kid's lifespan/existence by donating one of your own body parts to this poor kid? Also, please assume that, in my second scenario here, this poor kid (for whatever reason) actually preferred to die than to receive a body part donation but his or her parents overruled his or her own desires in regards to this and accepted this body part donation in his or her place. In addition to this, please assume that, in my second scenario here, the taxpayers will be the ones who will have to help financially support this poor kid if you yourself will refuse to do this.

Basically, I myself see both of these scenarios in these terms:

-In both of these cases, you cause someone to have a longer lifespan/existence than he or she would have otherwise had. -In both of these cases, you are responsible for the fact that this person's current state is existence rather than non-existence (after all, death is simply a return to non-existence). -In both of these cases, the taxpayers will have to help financially support this child/these children if you yourself will refuse to do this.

Indeed, what exactly am I missing here? After all, my current perspective in regards to this causes me to conclude that if parents who have consensual sex should be forced to pay child support for 18+ years afterwards, then I must also believe that people who donate body parts to children (or at least to poor children) should at least have a moral obligation to financially support these children afterwards (imposing such a legal obligation on such people might discourage body part donations, which is certainly something that we don't want).

Also, I would like to point out that, in regards to legal paternity/maternity, the law in regards to this can (theoretically) be changed so that a child's biological parent(s) will be able to stop being this child's legal parents (without adoption; indeed, this already appears to be the case for both sperm donors and egg donors).

Frankly, what exactly, if anything, am I missing and/or forgetting here? Any thoughts on this?

Indeed, is there a meaningful difference between causing someone to exist in the first place and extending someone's existence/lifespan? Any thoughts on this?