now I understand, thank you Jeff!!

You miss the point of the joke. The words, halter is now a kind of dress worn by women. So it meant a woman clothes is being removed. A bridle and reigns are strapping used to control to the horse to guide its movement according to the rider. So unbridled means "uncontrollable" and "reign" means to control. So if you rewrite the phrase using other words, such as, "... They quickly removed their CHEMISES there, with nothing to CONTROL them in, it would be a night of UNCONTROLLABLE passion..." will not be appropriate for a HORSE in the bed with sweat coming from his brows. The title of the book, "50 Shades of HAY" will also be not funny at all.

Oh...I have not read " 50 shades of Grey" yet. Did the author use "halter","unbridled" and"reign" in her book to describe certain scenes you know... so that those words got the new meanings?

@Max: No. The words, "halter", "unbridled", "reign" are words that comes from something to do with horses originally. Now they acquire new meaning.

it it because the words using in " 50 shades of Grey " are very commonplace and childish, especially when it comes to the sex part? About the picture...only the horse would consider the episode hot?