Underwear?

A priest explains why Mormons are not Christians:

Your original assertion was correct: Mormons are NOT Christians, because they do not baptize in the name of the Triune God. This is the basis upon which all Christian demoninations (including RC's) baptize and recognize each other's baptism. For example, if a baptized Methodist or Evangelical Christian wanted to enter the Roman Catholic Church, we would not re-baptize that individual because we recognize a common baptism by all Christians. The same cannot be said for Mormons.

A reader sets me right on one thing:

Just so that you know Andrew, most Mormons hated the Passion of the Christ.  Mormons have a very strict policy of no watching R-rated movies. Those who did generally hated it. You are right - Romney will fail over a few things: racism, Joseph Smith, and temple ceromonies (including his Mormon underwear). Imagine him trying to explain the underwear issue to the public.

I had no idea about underwear. Is this an urban legend? Or is there something to it?