Of Mike and Men: It was allowed as an answer on the US census a few years back.
It's not a true religion meant to belittle true religious beliefs by being completely ridiculous with the sole intention of then saying:
"It's no more ridiculous than Christianity/Judaism/Whatever! Ha Ha Ha."
http://knowyourmeme.com/memes/cm-punk-is-not-impressed
Take that as you will.
- Justin