Inspired by a Tumblr post explaining Castiel's line in the episode "99 Problems." Setting is probably either late season 5, or during season 8. You pick.
PS: This has been slightly edited since its original posting. I just reworded a few sentences.
PPS: I plan on rewriting this.
Over time, Sam had come to realize that angels, robotic as they seemed, actually did have a sense of humor. It was just more subtle, and took an informed mind to understand.
He was certain that by now, Cas was definitely capable of not scaring Dean single every time he showed up. And, though the angel remained stone-faced, Sam was sure there was a glint of amusement in his eyes whenever the elder Winchester nearly jumped out of his skin.
Besides being pranksters, angels also apparently liked jokes. They just required some further knowledge to understand.
After the Whore of Babylon, Sam had (naturally) done some research on the Enochian phrases used as false exorcisms. After a bit of digging, he found his answer.
There was a superstition during the Middle Ages that goats would "whisper lewd sentences in the ears of saints."
Therefore, the phrase, "You breed with the mouth of a goat" was actually an expression for "You talk dirty when you fuck."
Some things really were funnier in Enochian.