Quote Originally Posted by Wikipedia
Also called the lesser apes, gibbons differ from great apes (chimpanzees, gorillas, orangutans, bonobos and humans) in being smaller, exhibiting low sexual dimorphism, in not making nests, and in certain anatomical details in which they superficially more closely resemble monkeys than great apes do. But like all apes, gibbons evolved to become tailless. Gibbons also display pair-bonding, unlike most of the great apes.
I have personally witnessed several monogamous pairs of Gibbons at my local Zoo. Not to mention that there are several birds that form monogamous relationships. You must have more research before you pronounce an over-arching generalization on nature.

-Sin