Seems unfair to downvote you just for misunderstanding, that was sarcasm friend.
I'm impartial to downvoting as to reply literally doesn't usually add to the conversation, which is generally the point of downvoting. Losing karma shouldn't hurt your feelings.
Agreed, this is why I didn't want to take a hard position on downvoting someone for missing the sarcasm. We're in a public forum which crosses cultural boundaries. I see the argument for both sides.
Although I lean more towards downvoting as the majority of people on HN will get the joke and it's an opportunity to learn for those unfamiliar with the mainstream culture.