1) Koran - [4:148] GOD does not like the utterance of bad language, unless one is treated with gross injustice. GOD is Hearer, Knower.
2) Talmud - [Ketubot 8b] Even a heavenly decree for seventy years of good can be reversed if one perverts his mouth with improper speech.
3) Bible - [Ephesians 5:4] Neither should there be vulgar speech, foolish talk, or coarse jesting – all of which are out of character – but rather thanksgiving. 5:5 For you can be confident of this one thing: 8 that no person who is immoral, impure, or greedy (such a person is an idolater) has any inheritance in the kingdom of Christ and God.
4) A gentleman has no skill in trifles, but has strength for big task: the vulgar are skilled in trifles, but have no strength for big tasks. - Confucius, Analects, c.400 b.c.
If you're religious, the prohibition is based on Genesis 38, in which a fellow called Onan avoided his familial obligations towards a woman, having sex for pleasure but pulling out to avoid conception. This irritates God, who promptly strikes him dead. http://en.wikipedia.org/wiki/Onan has an in-depth explanation from multiple doctrinal perspectives.
I think the GP has a fair question - the stereotype of people who label themselves 'Reverend' includes a considered reluctance to use profanity. It's not saying that reverends can't do this if they're 'real', but it is far enough out of the stereotype to ask just what kind of denomination is the author a reverend for?
Whether or not it's "written" somewhere doesn't mean that there isn't a stereotype as to how 'holy' men (it's always men...) should behave. Witness the confusion when Westerners got into Indian religions in a big way, then couldn't quite square the way some gurus shagged themselves silly - 'holy' men were 'supposed' to be celibate, if not in actuality, then at least in behaviour. It isn't the done thing to have your Anglican priest bragging about his last shagging session with his wife, for example.
Or is the author using 'reverend' as a joke appellation, without seriously being one?
Okay, that's a different view — one that I am more prone to agree with, although I feel the language did lend a visceral tone to the piece, which seems appropriate given the subject matter.
1) Koran - [4:148] GOD does not like the utterance of bad language, unless one is treated with gross injustice. GOD is Hearer, Knower.
2) Talmud - [Ketubot 8b] Even a heavenly decree for seventy years of good can be reversed if one perverts his mouth with improper speech.
3) Bible - [Ephesians 5:4] Neither should there be vulgar speech, foolish talk, or coarse jesting – all of which are out of character – but rather thanksgiving. 5:5 For you can be confident of this one thing: 8 that no person who is immoral, impure, or greedy (such a person is an idolater) has any inheritance in the kingdom of Christ and God.
4) A gentleman has no skill in trifles, but has strength for big task: the vulgar are skilled in trifles, but have no strength for big tasks. - Confucius, Analects, c.400 b.c.
5) Buddism - "Right Speech" : https://en.wikipedia.org/wiki/Right_speech#Right_speech
Note that I'm not sure where to look for other religions of the world.