Well, if we're going to be technical, you don't alter sexuality (mindset) with surgery, you alter sexual organs. And that wasn't so much a change in sexuality as a change in ability to reproduce. :X Also, "surgery in a nature" seems like a bit hard a precedent for.
It depends on your yardstick; from an evolutionary perspective, eunuchs are still extremely new, and I'm unaware of examples in other species. Also, most eunuchs had it forced upon them, so it's not really a fair comparison.