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.
http://knowyourmeme.com/memes/butthurt
That matches my understanding of butthurt as an overreaction to being chastised or shamed.
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?