I privilege the status quo

But what if a majority of people agrees on a change? How can we decide how large it must be to have its way? It's a troubling question for me because in political systems such decisions are usually pretty much arbitrary: why require a 70% supermajority vote and not 60% or 80%?

Unless the required supermajority is very near 100% (and has good reason to be so), I'm too afraid of the tyranny of the majority and would prefer a system where each voter actually controlled the proportion of nature that he or she is voting "for".

This is a mischaracterization of many, if not most, pregnancies.

I acknowledge this problem, but it doesn't change my conclusion.

The fact that most future people haven't been conceived yet doesn't seem meaningful to me, assuming you're not expecting species extinction next week. Does it matter who they are exactly, as long as they're people? It doesn't seem so to me.

But they're not people. They're possibilities. They do not exist.

I accept the following reasoning: the future world will contain many new people no matter what I do. I prefer a future world that's nice for them. That makes perfect sense.

The problem for me comes when people imply that ownership of parts of nature (e.g., tracts of land) should be forbidden. For instance you said,

messing with nature is going to be stealing it from somebody who was entitled to its being left alone.

I don't accept that people who don't yet even exist are entitled to a piece of nature I'm using today (if I don't own it). I don't intend to die before these future people are born, so I'll have to share with them. The more new people are born, the smaller my remaining share - even if it's a time-share or some such instead of a literal piece of the property. I'm willing to share - after all I didn't create this land, so it shouldn't be mine exclusively - but only up to some limit.

If the world population is X, and the small country of Breedia invents a molecular manufacturing technology that lets them convert all their mountains into 10X small children, I hope they won't become entitled to nine-tenths of the world's resources. I'll feel sorry for the children and I'll do everything I can to find them a place to live that's not too much at the expense of existing people, including myself. I'll also vote for anyone proposing a singleton that would prevent the neighboring country of Multiplia from doing the same thing next year.

Why do you assume that making the future a nicer place to live will also make it more crowded?

It's the nature of population that it grows until it encounters a limit - either of resources, or cultural. I hope that future humans will breed more in the presence of more resources, and less in the presence of less resources, but I don't fully trust this will happen.

Suppose the number and timing of children were limited only by the delay of nine months' pregnancy, and the costs of raising children were negligible. I expect the world population to rise rapidly and without limit in this scenario.