marcparis:

She has kept this a secret for years, and wrongly assumed that her ADULT son could handle hearing it.

If I understood the context of the letter, the mother revealed her donation because she thought that her son was contemplating being a sperm donor. See, that's not nuts, that's good parenting. When your kids are in the valley of decision, it's honest to share your own struggles with the same decision and the results thereafter, it helps the next generation make an informed choice.

The son freaked out. Maybe that's not the re-reaction you think you would have made, but it's valid. The real question is: Will the son donate sperm?