I'll fess up.

1. I should never label anything as possible homework?
   Nope, but one should be cautious.

2. I should have been smart enough to notice this one wasn't?
   Yup. Granted, everyone makes mistakes, but given the relative rudeness of saying "Homework" (Which is absolved if you're right), this was a negative. (The giveaways were that the poster had achieved rank, and a hash slice isn't commonly used)

3. I should have replied to the original poster privately to first determine this?
   Nah, If this had been an anonymous poster I'd have done the same thing.

4. There's no positive content, so only the people that routinely vote all of my posts negative won out?
   I'm not in that group, so I can't say. Probably, as time passed, this became more prevalent. I voted pretty early on.

5. Something else I've completely blanked out on?
   Well, I don't recall getting any money from you ....:)

In general though, I think most people would go back and drop an Update in their post once the error had been detected. You didn't, so the negative votes kept rolling in (because the voters have no idea the post is negative until after they vote, unless they checked Worst Nodes)

At this time, your post has a -5, which means 5 more people thought you were wrong enough to be negative than thought the post was good enough to be ++'ed. Given that we all pretty much agree that your post was incorrect (however well intentioned), this doesn't seem surprising.