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):
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.)