On a more serious note than my previous post...

Let's not forget that most animals mate only when the female is in estrus. At that time, she is producing chemicals and exhibiting behaviors that excite males of her species. For cross-species mating to occur naturally, the chemical and behavioral cues from a female of species A must be similar enough to those normally given off by a female of species B to arouse a male of species B.

Humans are the only animals who willfully have sex with members of other species. When it occurs in other species, it's due to chemically-induced confusion.