A fetus is a living think with full human dna that unless we interfere, will become a person with full moral standing who wants to live. There is no rationally denying this fact and whatever ethical implications they hold. Killing a fetus is acting to cause a person's non-existence, which is also true of murder. The difference is that since we decide (reasonably IMO) that the fetus is not a person yet and thus has no rights to live, it is not murder or at all illegal. But ethically, its about as close to harming as you get, without really qualifying (closer than killing an adult chimp).
Eggs and sperm are living things with human DNA and the potential to become a person should the right conditions be provided. The argument you outline is the same one used by the quiverfull folks to say that
failing to have sex when you are ovulating deprives a potential human of life and is immoral. There's no difference in the argument. In neither case is an actual "person" already present, just the potential.
There is a massive difference. Neither the sperm nor the egg have the complete biology that would allow it to develop into a person. Also, you completely ignored most of my argument about the critical role of causality and inaction versus action, where I explained how failing to act in order to do the things needed for a person to exist is not at all the same as acting to prevent a person from existing that otherwise would if you did not engage in specific acts to stop the process. It is the same difference as failing to hang out at the Golden Gate bridge everyday, so you can talk each person who comes to jump out of jumping, versus going there in order to push people off the bridge.
In addition, there is no valid scientific basis for defining a human being based upon which side of the birth canal it is on. The idea that a "person" is so defined is purely a political definition of a "person" that serves (validly IMO) to demarcate the legal issues and recognize that the fetus doesn't have individual rights until it is no longer physically inside the mother's body over which she has the rights. Scientifically, a 6 month fetus and a newborn baby are far more in the same category of living thing than is a 6 month old fetus and a sperm.
It's a cycle of applying personhood to things that are not persons and it's not logical or consistent.
I have been clear that "personhood" is a purely legal construct and not what I am talking about. I am talking about ethics as they relate to the scientific and causal realities of killing a fetus just before it leaves the womb versus just after it has left. It is quite logical and consistent to impart the act of killing a fetus some moral status, even if it not at all illegal to kill it due to its lack of legal rights as a "person" and to the mother's rights as a person to control her own body.
If the person has not yet developed, any choices in deciding to get to that point of personhood do not affect any person who exists.
True, as long as you recognize that that whether a "person" has developed is not at all a scientific position, but a purely legal one.
But even then, the act of abortion has very direct causal impact and is essentially THE neccessary and sufficient cause of the person not existing in the near future who otherwise would have. The fact that no person yet exists is what makes it not legal murder, but does not make it an entirely non-moral issue for any remotely coherent and principled ethical system.
Choosing to be a biological parent when you know you have a certain debilitating disease is choosing to give that disease to a child. And one could argue reasonably, I think, that it would be immoral to knowingly choose to give a child Down Syndrome or any other dibilitating disease.
No, that is inconsistent with your stance that
"it is not possible to do anything to "her/him" before "s/he" exists". According to your premise, any choice you make, even one to drink and give the fetus downs' is not immoral. Allowing any fetus to continue cannot be immoral. Only after the baby is born does any choice take on any moral implications. So only the choice not to kill a newborn with Downs' could be immoral.
If this feels wrong to most people and likely to you, it is because it is the logical consequence of your utterly arbitrary moral dichotomy in which killing a newborn is one of the most heinous act possible, but killing the same organism 1 minute earlier just because it is still in the birth canal has zero moral implications.
Arbitrary cutoffs are what law is all about, but it doesn't fly with any sensible moral code.
I think sometimes people romanticize the idea that a "soul" exists with a "fate" and if you do something to prevent that pregnancy or person from getting "their" chance at life you have done something wrong to an actual being.
It isn't "fate", it's scientific causal determinism. If you didn't deliberately kill the fetus in order to prevent it from reaching a legal boundary to qualify under the law as a "person", then it would have become a person as a product of deterministic causal forces already set in motion.
But I agree with teh argument that there is no being to be done to, therefore it is not possible to do anything to "her/him" before "s/he" exists.
The part I bolded is false. There is no legally recognized "person", but there is definitely a "being" and if not already a "human being" by an reasoned scientific criteria, then as close to that as any being in the universe can be (and arguably closer than conjoined twins and born infants with major defects).
Basically, the parts of your argument that are valid apply at least as much if not more to adult chimps as to human fetuses. So, to be valid your arguments must apply at least as well to adult chimps (who unlike fetuses are not developing humans and have zero probability of reaching "person" status).
Just like with a fetus, killing a chimp is not killing a person and thus it is not murder, but doing it for reasons other than those seen as vital for human life would be widely viewed as unethical.
In addition, for your argument to be valid, it must also treat drinking and doing drugs while pregnant as at least as morally neutral if not more so than abortion.