Did a search and hoping this topic is not out of bounds for zity (am guessing not).
i have an ambivalence about this, both attracted and repelled by it. i put this in the psych corner because, for me, it is more psychological than physical.
i was introduced to the idea of nullo by a Dom/Top guy who explained to me that the biggest gift a partner could give to Him was his cock and balls. He said He would never ask it or demand it, which had the effect of planting a seed in me and causing my ambivalence. I.e., a part of me now desires to please a guy in that way. To me, nullo is an ultimate form of expression of bottom/sub, giving up ones ability to penetrate sexually... at least, in that way.
Also, the biggest component for me of D/s ,Top/bottom, is the dynamic of the Dom having control over both His and His partners sexual pleasure. In part, the idea of giving my penis but retaining my testes makes more sense to me than the Eunuch thing, the other way around. i don't identify as female, so that is not a part of it. The idea of retaining my hormones and drive that goes with it, while giving up the primary male expressions does capture me though, especially submitting those to a guy who wants them.
feel like i am rambling, this one is difficult for me to articulate.