"I want to spend a million years having sex with cat-girls after the Singularity."
Eliezer, I think your mental picture of the cat-girl scenario is a straw-man.
Suppose you had done all the philosophy right, and you knew exactly what eudaemonia meant for you, and then designed a cat-girl. What would that sort of cat-girl be like? Different from your friend's imagined version in more than a few important respects, I'd wager.
"""If you take that fun and redirect it to something that isn't socially entangled, if you turn sex into an exclusively single-player game, then you've just made life that much simpler - in the same way that eliminating boredom or sympathy or values over nonsubjective reality or individuals wanting to navigate their own futures, would tend to make life "simpler"."""
For starters, a properly designed cat-girl mightn't necessarily make things "simpler" at all. I think the real problem is that you're imagining cat-girls which are too easy. Does playing chess against a computer still count as a "single-player game"? The computer really is trying to beat you, even if it isn't sentient or trying to beat you in the way a person would. And it's conceivable that the world's best chess-player might find playing the computer more interesting than playing other people, and he'd be right, because the games he plays against the computer really are more harmonious chess-games....
If you actually tried to optimize the programming of a cat-girl for the eudaemonia of the cat-girl's partner, you'd be searching mind-space for peaks of some function. The higher peaks mightn't be inside human space, or even something reachable by minor tweaks to existing members of humanity. It's not that unreasonable that this would be so: you already mention the potential for small tweaks to fix "the problem." I would add that our parallel empathic architecture is a kludge, and therefore there should be ways of behaving that look more human than the actual ways people behave. In any case, the human part of mind-space is tiny.
And if the fitness landscape is anything like the fitness landscape is for, say, intelligence, those peaks could be ridiculously high compared with anything in the human space.... Therefore, if you insist on not replacing human partners, the losses, of, not sex, not love, not praise, not challenge, not any such superficial indicator, but of actual quality of life, whatever that phrase actually means, could be staggering.
I'm not saying I wouldn't be saddened if the human race went its separate ways. The amount of eudaemonia gained by ones partner actually being human may really be that big. I just don't know. Maybe it really is worth an absurdly huge sacrifice to keep the human family together. But don't make the sacrifice look smaller than it actually might be.
PS: What the hell do you mean by the "east" and "west" sides of the galaxy? "West" is the other way on the other side of our planet! Someone is thinking like a Terran.