Saint Benedict wasn’t sparing quadrupeds out of a sense of kindness to our animal friends exactly; meat was thought to make people lusty, as everyone in the Middle Ages had been around animals enough to have seen quadrupeds copulate at some point in their lives. Eating animals brought thoughts of animals to mind, which brought thoughts of animals mating to mind, and an entire lifetime of celibacy was difficult enough without being made to think sexy thoughts at every meal.