As a child at the zoo I was taught that one is not allowed to view animals while they are copulating. However as a pet owner I have learned that the exposed genitals of animals do not prohibit a person from reciting a blessing or learning torah. I would like to know if it is in fact true that a person may not watch animals fornicate and what the source of this prohibition is.
Rambam, Laws of Prohibitions on Relations, 21:19 (or #20, depending on your edition):
וכן אסור לאדם שיקשה עצמו לדעת, או יביא עצמו לידי הרהור ... ולא יסתכל בבהמה חיה ועוף, בשעה שמזדקקין זכר לנקבה; ומותר למרביעי בהמה להכניס כמכחול בשפופרת, מפני שהן עסקין במלאכתן ולא יבואו לידי הרהור.
A man should not bring himself to arousal ... [gives a few examples] ... and similarly he should not stare at animals while they are mating. Professional breeders, however, are busy focusing on their work and this causes no arousal for them; hence they are even allowed to directly couple their livestock.
So it's not erva per se, it's "avoid the suggestive."
(As the commentaries point out, Rambam is codifying Gemaras from Avoda Zara 20 and Bava Metzia 91.)