The fact is that homosexuality can be found to some extent in most species of animals. But it is inappropriate to base human behavior on lower animal forms as your statement demonstrates.
From a purely natural stand point, the human male and female sex organs are complementary and designed to function with each other—not with some other part of the human anatomy. They are not designed for use with other species and are genetically incompatible with them.
Incest is naturally problematic because it can result in abnormalities in offspring because the blood lines are too close.
Heterosexual relationships alone express a complete continuity between the human body and its sexual expression. Attempts at sexual intercourse outside of the heterosexual model can only be at best, an accommodation.
Incidentally, I never ever thought that I would have to defend heterosexuality as being natural. Never did I think that our culture would be so off balance!
Fr. Vincent Serpa, O.P.