From "Homosexuality in America: Exposing the Myths" (American Family
Association, Inc.)
Myth #2 - Homosexuality occurs in the animal kingdom and is therefore natural....Cannibalism is found within the animal kingdom, but surely no one would want to argue that cannibalism is "natural" and therefore should be allowed among humans. So, even if it were the case that homosexuality was natural within the animal kingdom, one would be wise to be cautious in his conclusions about human behavior on the basis of this.
[Another] problem with this argument is that it is not proper to equate human homosexuality with the seemingly "homosexual" acts that occur among some animals. Brian McNaught, in his video On Being Gay, makes the outrageous statement that "In every species of mammal, from the sea horse to the baboon, there is homosexual behavior, whether in captivity or in the wild." This is clearly wrong. Human sexual activity is quite different, even physiologically speaking, and certainly emotionally and spiritually speaking, than that of animals. As Robert Knight comments:
Anthropologists and sexologists interested in promoting the sexual revolution often interpret the sexual behavior of animals as analogous to human behavior. Scholars of animal sexuality rarely make that mistake because they know that animals are aroused in entirely different ways than are humans...
A study done by the U. S. Department of Agriculture on the sexual behavior in sheep is sometimes cited as evidence of homosexual behavior in animals. According to interviews with the researcher, however, there was absolutely no relationship between the conduct of the sheep and human sexual conduct...Thus, to argue for homosexuality by appealing to the animal kingdom is futile.