I think this is analogous to the philosophy behind "duty to report" type laws. If you discover -- even through a completely unrelated activity -- harm being done to someone, it is your ethical responsibility to report it to someone who can help. Obviously
some amount of discretion is necessary, as some things are sensitive enough that reporting in the wrong way, or to the wrong person, could cause the situation to be worse, but as a general rule, if you see something bad going on, you should try to make the situation better if you're able.
I think OP acted entirely appropriately.
To address a couple specific points:
> As an admin you have an obligation to your users to not be nosy
In the free-wheeling academic sense where your users are more of a community, sure, I think that's the accepted social contract. In the workplace, not at all. While I'm not a fan of employers spying on what their employees do on the employer's network and hardware, I fully appreciate that it is their right to do so, and in some situations, for some purposes, I might even agree with its necessity.
> reading the plaintext yourself for fun then scurrying to HR is kinda a slimy thing to do
I don't think "fun" had anything to do with it, and reporting a likely case of sexual harassment, regardless of how the information was obtained, is never "slimy". Quite the opposite.
> Just because you have the ability to peek into the CFO's mailbox and see what everyone's salary is, doesn't mean you print out the spreadsheet and take it to your boss demanding a raise.
That is indeed slimy, unethical, and likely a violation of company policy, but that is not even remotely the same as what the OP did.
> if you got in trouble for playing Farmville or whatever while sitting on the toilet at work, which they found out about by installing cameras in the stalls
Also not even remotely the same. Any reasonable person would agree that cameras in bathroom stalls would be a gross violation of privacy (and probably illegal).