Because thinking about it, it’s counter productive to reproduction. So I imagine the Catholic Church was highly against it back in the day.
You might have better luck with this question if you narrow it down to a specific region and/or time, such as:
What were the views of the Catholic Church on oral sex during the Middle Ages?
Also, you might find this previous question to /r/AskHistorians of interest: Was oral sex an invention of modern western Europe? For instance, did Native American/First Nations people practice oral sex that we know of? And what about East Asians, and other non Europeans? [NSFW] that included some excellent answers by /u/CommodoreCoco (South America), /u/lordtiando and /u/bigbluepanda (China), and /u/sunagainstgold (Rome).
From that post, here's a picture of a Peruvian ceramic from 800 AD (or earlier) showing that oral sex was definitely a "thing" back then.