Since his grandmother died from Huntington's Disease I think Steven has been constrained and lives in fear of either getting it or someone in his family gets it. And if he gets it, I think he wants absolutely no one to remember him by anything, so no one will feel like they lost anything, simply because of this Disease he may or may not get. That's why when he initially visits the doctor he persists in knowing about it, to see if he may get it. He is simply lying down and accepting his fate as a human being, and won't do anything and then blames it on fate.
I think its very similar to Oedipus because once Oedipus realized the prophecy was true he could not stand to do anything, to live almost. He does not want to do anything, and wants to just get away through exile. Oedipus just like Steven was fated at birth, when Jocasta had him, she knew that her child was foreseen to sleep with her and kill her husband. While Seagle is fated to live in fear and sorrow because of his families genetic disease that kills. The prophecy in Oedipus is Huntington's in It's a Bird.
Personally I do not think that the broken pieces of our childhoods are what can bring us down like kryptonite, because I think that even if our childhood was broken, we can overcome those broken pieces to succeed and prosper. And we can make sure that our children are not "broken" by childhood because we as parents can provide as good a childhood as we humanly can. My kryptonite would have to be my family, in the sense that if anything happened to them I would do anything to help them. If they ask for money in the future or any good, it will be hard for my to see my family suffer if I do not help them by giving them what they desire.
Steven's distaste for having children, to me, stems from his fear that the child could contract Huntington's Disease. I believe that is how much he lives in fear of the Disease so much that he will not risk a kid getting it.