Where is it written that a Reverend shall not use such language?
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.
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?
Section 14, "Unlawful Birth Control Methods"
Search the document for "contraception".