I am in the midst of preparing a lengthy post demonstrating how flawed the argument is. In the meantime, by serendipity I have come across the following story in New Scientist magazine, on the omnisexual appetite of the rough snail. Remember, that logically a single counterexample is enough to demolish an argument such as "all animals avoid same sex activity." This counterexample eliminates that claim at a stroke. Other counterexamples, which I will present shortly, will demonstrate the more modest claim that exclusive heterosexuality is somehow "normal". What is abnormal, in the global context, is exclusive, compulsory heterosexuality .
Males track females by following their mucus trails, and will attempt to mate with pretty much any snail they encounter, regardless of whether it is the correct sex or even the same species. They mate with males just as often as they do with females – though they do give up such copulations sooner.