"We currently have the ability to turn reproduction off. Yet many people go to extraordinary lengths to turn reproduction on. "
This is true because there are still many good reasons to have children. I don't see any of these reasons being certain and compelling by the time intergalactic travel is possible. We haven't even scratched the surface on what technology is going to do to economics and already the maternity rate in prosperous countries is below replacement. We may well expand forever but I don't think it's at all obvious.
"If even a small minority opts to reproduce and expand, they will have huge selective advantages over those who opt out of reproduction and expansion."
What if a small minority wanted to kill everyone? As technology increases (to the point of allowing things like an independent faction to do this), you have to assume there would be strong pressures and protections in place to prevent the sort of factionalism that currently dominates. And if a large, technologically advanced majority doesn't want you to reproduce I'd guess you are not going to reproduce.
I find all of these propositions questionable. It's not clear at all that they will need to (1) reproduce (2) relocate or (3) capture an absurd amount of free energy. We can speculate they might want to do any of those but the arguments that they won't seem just as strong.
I highly doubt there will be any disagreement about the merits and needs of colonization within a civilization capable of intergalactic travel - it will either be a good idea and they will agree or it will be a bad idea and they will agree not to.
Seeing no evidence of colonization (and knowing that if they all do it they If they will come into conflict with each other and risk their extinction) let's suggest they all decide not to do it is a reasonable possibility.
Then timtyler's point is easy to see: this isn't so much about doomsday as about a change in society that devalues reproduction and expansion. There may be very few to no more humans born after say 2300 AD. And that's because people don't need offspring to work in the fields anymore, don't fulfill their sexual needs like other animals, have incredibly inflated lifespans, etc.
Colonies and expanding populations likely become irrelevant once you approach the level of technology necessary for intergalactic travel. Quite possibly communication as well.
"Children have gone from being productive capital goods to consumption goods. I don't see any evidence that children are losing or will lose their value as consumption goods."
Wait -- the value of children recently changed fundamentally but we should expect no more change far in the future?
"I'm saying that the zero population growth faction will be a tiny minority by the time a civilization grows large and advanced."
This does not reconcile at all with current population trends of developed nations. The UN medium projection for 2050 has the entire world at 2.02. Go ahead and assume people still want children as consumption goods, data suggests that not enough of them want this to maintain even zero population growth beyond the current century.