Evidence? Know Your Meme documents it as related to spanking:

http://knowyourmeme.com/memes/butthurt

That matches my understanding of butthurt as an overreaction to being chastised or shamed.

"butthurt" is the pain you feel in your anus after being anally raped.

If you scroll down on that knowyourmeme page that you linked, you will see a set of memes in the "Buttfrustrated" category. The Brony meme has a set of insults that reference the anal rape: "Im sur thers sumpony who can help fill ur ass with semen laced hrut".

The percentage of cases in which a person is referencing "butthurt" to indicate a spanking is vanishingly small. Play the game "League of Legends" and complain when you lose. Ask people what they mean when they claim you must be "butthurt".

Did I just type those words?

I could believe some people use it that way; I just disagree that's the primary meaning.

Yep, you just said that!

No, it's not. It references rape, but is not synonymous.

Its current definition is roughly synonymous with "unreasonably upset".

I see, I've suggested a very simple change to a single word of your comment which would make it more inclusive to the community, but you have refused to change it and instead taken a stand of principle.

I now have the choice of respecting your decision, or pressing the big red button to incite an internet mob upon you and score one for radical feminism.

I'll let you know my decision shortly.

You should found a company and then not hire that guy but then write a blog post about how you would totally fire that guy.

I think it's a bit ironical that you seem to be acting "butthurt", threatening with internet mobs and such (and the gloomy prospects of someone losing their job if they don't bow down to your pressure).

Unless you are a rape victim you have no right to determine what is and isn't acceptable usage of the term.

Please respect the triggers of other people and check your privilege.

butthurt isn't slang for rape, and you don't get to decide what the community understanding of the meaning is. Which is someone unreasonably upset.

source: my understanding, peer understandings, and every post on urbandictionary