Unfortunately, I don't have time to read and respond to all your posts right now, but I just want to add to this one. Having multiple kids, you're right, OF COURSE I have less money per child than someone with only one. OF COURSE their lives are very different because of their siblings. But this multiple kids analogy is severely flawed in another respect that is at least as important:
We are meant to raise kids with the expectation that they grow up and go out on their own some day. Even apart from the sexual and romantic aspects that make the child analogy totally ridiculous, we do not enter a romantic relationship with the idea of eventually sending that person away, out on their own, to live a life apart from us.
Raising children is about enabling them to leave us, whereas a romantic relationship--at least to most people--is about finding someone who will stay with us, and with whom we will stay.
And I agree with you on that approach. Yes, we raise kids with the expectation that one day, they will leave the nest. That's normal, and completely natural. And really, it even more so divides the analogies of partner vs parent/child. That's why the loving multiple children analogy as a means to explain the ability to love multiple partners is also severely flawed. Keep in mind, that the context I made the comparison (adopting a child to pick up the slack of a current child) was not to suggest that having more than one child was a bad idea. However, it is important to recognize that children often do feel a deficit of love. And telling oneself that they'll eventually get used to it is not a noble justification.