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Yeah, some animals will cannibalize their offspring or mate, some will rape (if it can be called that) other species, or kill seemingly just for sport

Yes, sometimes animals kill when they're NOT hungry, they torture or "play with" their victim, really drawing out the killing process, they certainly have intercourse without consent, and yes I've seen a cat sodomize another male (his brother, actually). But here's the thing:

1. They are irrational animals. They are brute beasts, lacking the use of reason.
2. God gave them no commandments. 
3. They do what they feel like, act on instinct, and live in the eternal "present" being completely without the use of reason -- so they don't obey any "laws" per se.

But here's the main point. HUMAN BEINGS ARE NOT ANIMALS. If you think humans = animals, then I have nothing further to speak about, because our views are irreconcilable. If we're just cosmic accidents of the universe then YES, morality doesn't matter. It's the law of the jungle all the way. Do what feels good. 

It's only with God and creation can you impose ANY KIND of morality upon the world. But common sense says that we should be moral. We all have a conscience telling us what is Right and Wrong. Ergo...

As I mentioned above ... what they're trying to assert is that sodomy is not contrary to nature, because it can be found in nature.  Whether it's sinful or not is refuted by pointing out that we're not animals.  But whether it's a sin contrary to nature is not addressed with that argument.

I make two arguments above (one similar to that of another poster) ... and then one more.

1) Some things exist in nature that are a corruption of nature, e.g. birth defects.  If someone is born without eyesight, it's not what nature (for us = God) intended but is nature gone wrong or corrupt.

2) So contrary to nature means contrary to what we can infer that nature intended.  Nature intended that human beings have eyesight and being born without it is still contrary to the intention of nature that we can infer.  Similarly, it's clear that the reproductive organs are meant for ... reproduction and procreation, and just because there's some corruption of that intent doesn't change the intent.  When rational human beings act in a manner contrary to the intention of nature, that's what contrary to nature means.