Functionally, there’s no difference between pro-life and pro-choice. It’s just quibbling over the specific demarcation point for when a new human has been created.
If you take a few hair cells from a person or two, turn them into pluripotent cells, turn one into an egg and another into sperm, introduce them to each other, implant into a real or artificial womb, and let the egg grow to maturity, then at the most extreme level you’re saying that you’re killing potential humans every time you let any of your hair cells, skin cells, or any other cell from your body go to waste without having had the chance to become a new human. You are doing less than is scientifically possible to be fruitful and multiply. Scratching an itch is murder.
If you want to say that humanity begins at the time that the sperm and egg are introduced to each other, then you still have to decide where that occurs. The instant some one of the sperms touches the outer wall? When the first sperm who will successfully get through the wall, touches the wall? When the sperm makes it inside? When the nuclei of the two cells first touch? When the nuclei fuse? When the egg first splits into two cells?
If our nuclei fuse and the egg begins to split, and we’ve declared this as a human, then we should expect all of the laws of humanity to apply to this person. For tax purposes, you have another dependent, while this child is still alive. The child should be granted a social security number, before ever being born since it is a human and US citizen. If, at this point, the human dies then it should be treated as the death of a US citizen. If someone caused that death, it should be treated as a murder. If that’s not the intention by most people who lean towards this end of things, then I’m somewhat skeptical that there’s a true belief by most people that this is where humanity begins.
But likewise, that would be true for later cutoff points as well (e.g. at some trimester).
Based on our laws, the universal agreement on humanity seems to come at birth. Most pro-choicers still opt to say that we’ve created a human before that point, so it’s arguable that there’s as much self-deceit in the pro-choice position as in the pro-life position, from how we’ve all voted and decided when the question has been asked in more indirect ways.
From humanity’s history, it seems to have been common to kill babies soon after birth or to let them die within the first year or two, if the family couldn’t afford to support them. Often, their corpses seem to have been tossed into pits and discarded of without much fanfare. Our primitive brain doesn’t seem to form a strong attachment to them until later than this. (Of course, our primitive brain also enjoyed war and cannibalistic raids on rival tribes, so we shouldn’t necessarily hold this up as a reasonable standard.)
From the standpoint of legal consistency, you could argue that the principal factor that makes a human into a human is our abilities of the mind that we have over all other animals. For those animals who lack those abilities, our laws allow us to dispose of them largely at will. Very young children still lack some of these abilities and, theoretically, artificial intelligence could come along that has the full talents of the human mind or even surpasses us and it would be murder to end those lives. A consistent legal framework for all lifeforms, natural and artificial, might make the division on this standard.
Ultimately, there’s probably no true logical argument for any demarcation point.
I’d probably vote that people should be free to have their own standard and that we should all a) be willing to live with that truth, and b) be free to move to another geographical region that shares our standards, if our beliefs require it. So long as people can move their place of residence freely, I don’t see that anyone is having their rights taken away. They chose to live there.