I am one that is all for neutering/spaying.
But I think your worries ARE justified. It's always scary when putting anyone under. However, the operation is so routine and many Yorkies even under 3lbs are spayed/neutered every day. That didn't change me from being scared to death while Jackson got it done, but I knew it was the right thing to do. I didn't want a marking dog, I didn't want a dog who humped all the time, I didn't want a dog who could potentially turn aggressive as he matures, I didn't want a dog who would wander constantly trying to find a female in heat. It's stressful for unaltered dogs who cannot mate.... because they have that need to mate two times a year when the females are in heat. And if they can't, it drives them crazy! I just wouldn't want to put my little guy through that. I'm sure there are unaltered dogs who don't portray all or any of those traits I listed above, but I would never own a male dog not neutered.