Personhood is a legal category and an assumed moral category that policies can point to. Usually, the rules being argued about are about the acceptability of killing something. The category is used differently depending on the moral framework, but it is usually assumed to point at the same objects. Therefore disagreements are interpreted as mistakes.
Personally, I have my doubts on there being an exact point in development that you can point to where a human becomes a person. If there is it might be weeks after birth.
Personhood is a legal category and an assumed moral category
Both of which are mostly based on examples and past decisions/precedent, rather than scientific or operational definitions. Thus,
it is usually assumed to point at the same objects. Therefore disagreements are interpreted as mistakes.
is itself a huge mistake. There is absolutely no reason to believe that any given legal framework agrees on any specific with other legal systems, and even less that moral systems would agree with each other or with legal systems.
Why stop at gametes? Consider a person a million years from now, most of whose ancestors are not yet even a gleam in their ancestors' eyes. They're still a potential future person, and we should still think about their welfare, to the extent that the effect of our actions now upon them are clear (i.e. mostly, we shouldn't go extinct).
You didn't discuss Utilitarian population ethics. But pretty clearly, increasing the population beyond what the available resources can ever support and bringing about an unavoidable population crash due to starvation and resource depletion (as some biological species do, such as locusts) does not maximize total utility. So under Utilitarianism you can both consider a gamete, or even gametes not yet produced, as a potential of a future person who would have utility of their own, and also (in appropriate situations) believe that use of birth control increases total utility.
Masturbation is murder!
Your deliberately overstated argument only applies to males.
More practically, the number of gametes any human (male or female) produces is a lot larger (especially for males) than the number of children they could possibly raise. The supply of gametes is not generally the limiting factor in how many children a couple can have.
Are animals persons? Do they have the same moral status as humans?
I’m sure all of you LessWrongers are aware of the argument from marginal cases.
For example, a chimpanzee that commits an act of rape or murder against a human isn’t seen as evil, or held morally responsible for their act. So perhaps moral agency is what makes humans persons.
But neither does a newborn or impaired human, so they can’t be persons either.
One objection to this is the notion of potential capacities. A newborn or mentally ill person may not have the capacity in the moment, but when fully developed and functioning properly, they will be fully morally responsible, rational autonomous agents.
But this logic can extend to before birth. Foetuses, embryos, and even zygotes could be argued to be potential moral agents.
Taking this reasoning to its absolute logical conclusion, we can even say that gametes are persons.
Every sperm is sacred! Masturbation is murder!
If this reasoning is correct, we might have to outlaw contraceptives, to prevent the destruction of innocent lives. I don’t know how we’re going to enforce a law against masturbating though, perhaps by installing surveillance cameras in every house?
A rebuttal to this might be that you or I were never a gamete. Sperm and eggs are body parts of the parents, so we cannot reasonably argue that personhood begins any earlier than conception, when a distinct individual organism first comes into existence.
Are you identical to your organism though? One could reasonably argue you are your brain or mind, in which case personhood begins later in pregnancy.
It seems then there is at least two elements of personhood.
Continuity of personal identity over time
Actual or potential traits and capacities
Perhaps there is even more factors we must consider when determining the moral status of entities.
I made this post for two reasons. As a conversation starter, and to have a bit of a laugh at absurd lines of reasoning.
I don’t know or claim to have a definition of personhood, but I think this post might be helpful to frame people’s thinking on the matter.