Are "Unnatural" sex acts unnatural?

A while ago I was shown a picture of a dog mounting a cat. I thought it was a funny picture in a twisted sort of way. But, like I said, that was a while ago. Well the other day I was over someone's house for a BBQ. We had finished eating, and were sitting around in the living room talking. I look down as the cat entered the room. Wouldn't you know it, but the dog walked up and mounted the cat. The cat meowed but did not move away. The owner told the dog not to do "that" to the cat (like I have told you before).

Interesting insight number one: It was obvious by the couple's reaction that they had caught the dog doing this on multiple occasions.

Interesting insight number two: The cat never tried to move away.

Interesting insight number three: The dog was enjoying himself. Oh yeah!

Was this unnatural? Dogs and Cats having sex with each other. I think that Bill Murray said that this was one of the signs of the end of the world during one scene in Ghostbusters II. The cat didn't mind (small dog). The dog really didn't mind. I don't believe that this can be called unnatural.

Unusual, yes. Rare, yep. Unnatural? No. To be unnatural, the dog and the cat would have to have intelligence and reasoning. So, if you don't believe that animals (dogs and cats here) are intelligent beings, you probably believe that they act on instinct for the most part. Animal behavior, right?

This throws the whole heterosexual male on female mode right out the window. What is a natural sex act if the natural order of things is a cat mating with a dog? Yeah, yeah, I have heard the "SIN" argument. But calling something a sin is a lot different than calling something unnatural. Fine, we are suppose to rise above our natural tendacies. But what is natural?