I know that the points contradict each other, and that's fine. My first point was that it was highly unfair to the women, this was a statement of fact, because it is. The second poing states that yes, the alternative wouldn't be fair for the kids, but here its to point out that simply speaking the immortality was designed for a single purpose of giving Naruto lovers that won't die on him. That was the qualification that had to be met to get immortality from Kiyomi. They don't meet the conditions, they don't get immortality. That's as close to fair as one can come with an unfair system like immortality in place.
You call it cruel to leave the children mortal, I call it natural. People die, that's the way of the world. Mortal children born from immortals are in no way cheated out of the lives, they have the same lifespan as any other human, and probably a bit more since they'll probably get the Uzumaki-longevity. The immortality is a unnatural state, in no way does a child have it as a birthright just because parents had it. That's like saying the child of two mathematicians has to be brilliant at math and its a cosmic injustice if he's not. The parents have something, that doesn't mean the kid has to get it.
And yes, we humans are able to suppress our biological urges, and not every woman is wired to be a mother. But you have a group of 30-40 women, amongst them mothers. Are you really going to tell me that of these women from all walks of life, from young girls to old ladies with a second lease of life, not one would want to have a child? Really?
I think the point I disliked most about your model of immortality was that it automatically alienates every woman who decides to have a child from Naruto. As you said, Naruto doesn't want a child, so while the woman decides for one, for Naruto it will always appear as if the woman in question has made a decision against him. To have a child means to in the long-term reject a life with Naruto, every child to Naruto thus becomes a permanent rejection from one of his lovers.
Not only that, but Naruto, who doesn't want children, will through the setup never be able to be a truly good father. Because as an immortal, there will always be a voice in the back of his head when he looks at the child that says 'You. She abandoned me for you, she rejected me for you, it's your fault that she will die.' Even if Naruto tries to be a dad, that thought will always be there in the room. For a woman the choice for a child is a need, and a symbol of their feelings. For Naruto it's a rejection and basically the herold of death for the mother. Not only that, but humans are fickle creatures by nature, and the mother may one day, when she's old and her former coven-sisters still young come to regret her choice for a child, because now she's mortal and humans, even ones given immortality, tend to make decisions based on current situations and short-term benefits over long-term consequences. So as a child you are born to a father who never wanted you and at least subconsciously blames you for your mother's death, and a mother who may someday come to regret your very existence because you stole her youth and robbed her of eternity. That, Lemon Sage, is what I call being unfair and cruel to the child.
Author's response
The cruelty is born of the fact that its being born of the sentiment of "Tough Luck kid. I know you're going to grow old and die, but I want to experience being a mother." Also, as you point out immortality is an unnatural state which is another reason why Kiyomi made not having children a condition of maintaining it. You state that having children is a biological imperative, but the simple truth of the matter is we can and do choose to follow it. And yes I do believe that given the choice between potentially living forever and having children they would choose to live forever.
You see that Naruto might come to resent the child, but I think the opposite might also hold true should the child be born but not be immortal. You point out that their response should simply be sorry but it's not a natural state so tough luck kid. But don't you think a kid would resent the fact that he or she was born without such a gift. Plus, if being immortal is outside of nature, which I agree it is, then why would they still feel a drive to have future generations. That need to have your genetic material passed on is born of the fact that they do have mortal lives. But with the fear of death, mostly removed why would they still feel a need to have children? Furthermore, your entire last sentence is exactly why I don't think any of the women would choose to give up being immortal to have a child. One the man they are with has made his desire known that he isn't interested in having one. Two, their current situation is living a lifestyle of sexual indulgence which a child would throw a monkey wrench into. Three, as the other women continue to enjoy their happy Harem lives, the woman that chose motherhood would be changing diapers, and taking care of it as it grew.
You also describe it as stealing her youth and robbing her of eternity. But the truth is that she willingly gave them up. That is what making a choice is, do the two results necessarily have to be equal. No of course they don't which is why sometimes its easier to make a choice? Would she potentially come to regret choosing to have a child? Yes that is indeed a possibility. But that is why there should be consequences for making it. By making there be a heavy consequence it does make a person consider the results of making it. That to me is the difference between a person that lets their instincts rule them, or who is guided by intelligence.
Finally, in regards to your last sentence everything you laid out is also exactly why I think the women would choose to remain immortal rather than have a child. Having a child is an unknown proposition, it's what's behind door number two when you already know what you have behind door number one. Will some people still take what's behind the second door? Sure, but if you know it leads in death while you can enjoy eternal life with what you already have, I don't think anyone would find it a tough choice to make. Kiyomi weighted the choice so that the women would choose door number one. You might find that unfair, but as you point out in your reasons life isn't fair, and as the one granting the ability she is also the one that gets to set the price.