"The first person to live to [200, 300, 500, 1000] has already been born"

[The Grand Nudger]

In your immortality example..you did not present castration of those who selected to be immortal as an option, ie yourself...and you did not offer to be the meat on the campfire, yourself.  How about those?  Those would belay all of my doubts about you, and your morality, and your notions of a moral sense.  After all, if someone's suggesting castration, and that we should eat someone for our own survival.......

I would only propose castration to those who can't be stopped otherwise from procreating, as an alternative to jailing or killing them. I would only propose such a thing if the evidence showed for a fact that the man under consideration tried to procreate, did procreate or intended without a doubt to procreate(except in those cases where it would be deemed an accident or where it would be clear the man would never attempt to do such a thing again, which would mean his case would have to be kept secret in order for others to not get any ideas about getting away with it, and we would have to make sure that whatever caused him to want to do this only the one time would be studied as a cause in order to prevent it from occuring again).

If they didn't agree with your new immortal race, your new immortal paradigm...you would have them castrated.  That is inescapable.  You could achieve the same end by castrating yourself.  It's easier to argue the notion of castration-as-solution for overpopulation when you do not include those who would not consent to it, and it's particularly easy when you limit it to those who elect to be immortal, those who would otherwise be contributing to the problem immortality brings directly...those who benefit.  You have, instead..decided that you would castrate others...to solve a problem that -you- will have created.  

Total fucking no-go, dude...to me.  

Once again, I would only propose such a thing at all if the evidence showed beyond any doubts that the current resources don't allow for an increase in population. I would also study the possible effects of such an overpopulation in order to determine if it constitues a serious enough risk to have to be contained by such means, where no other means would do.

It doesn't matter how well you rationalize what you are willing to do.   If it;s a last ditch effort, the very back thing on your mind...then that thing is still the forced castration of -others- in support of your desire for immortality...and to ameliorate the problems that your desire created.