Robert, you keep arguing in favor of what Daniel Withrow has (rightly) termed a social contract based on mutual non-aggression. But despite some truly excellent posts in this thread, I’m still a little unclear on what exactly you’re basing this argument on. Are you saying that because animals cannot enter into a social contract, they do not deserve the same treatment as other humans, who are presumably capable of making such an agreement? If so, why do you still include infants and other humans who are 1) physically incapable of doing harm and 2) mentally incapable of entering into a social contract? You seem to be including these groups by proxy alone, based solely on their genetic inclusion in the human species.
On what do you base this belief?
As for the ability of animals to enter into a mutual non-aggression pact…well, I’m not so sure they can’t. Here’s an example: Let’s say you have a dog. Let’s say your dog is a large, powerful, and potentially aggressive breed, such an American Pit Bull terrier, or a Rottweiler. Such an animal is perfectly capable of doing you serious harm, even killing you. Yet it does not. You treat your dog well, love him and feed him and care for him, and he returns this with affection. In effect, you and your dog have agreed not to harm each other because doing so would not be in the best interests of either of you. You have entered into what is more or less a social contract with an animal you previously defined as incapable of making such a contract. Yet clearly the rules of such a contract are being upheld, and both parties are benefitting from the results.
According to your view, I assume (perhaps wrongly- if so, please correct me) that such a contract would not count because a dog is not a human, and therefore cannot make such a contract. (Although it is clearly behaving as though it had.) In such a situation, would you consider it immoral for someone to abuse their dog? Or would you find it immoral because that contract had been violated?
It’s been pointed out before, but I think it bears repeating: although animals are considered property under the laws of the United States, the US also sets forth specific laws about what you can and cannot do with your own property. You may purchase, own, and use a gun, but only in very specific circumstances and under very specific conditions. Where is the illogicality of the law stating that you may own an animal but may not do with it exactly as you please?
As to my own beliefs, any deliberate abuse, either physical or mental of any living being is immoral. The protection we extend to humans incapable of making a physical or emotional MNP should, I believe, logically be extended to all other creatures who are also incapable of making an MNP. (Before you start crying hypocrisy, yes, I’m a vegan.) How you treat non-socially-contracted humans (i.e., infants and the metally disabled) is in many cases the yardstick by which your own humanity is measured. Taking this to what seems to me its logical conlusion, extending our benevolence and protection to all living things would be in effect the most consistent human position.
Humans beings can in many cases be considered property, even in this day and age, and I trust no one needs a refresher about slavery or any other system of human ownership. Children are technically the property of their parents until the reach the age of majority, however that is defined by their society and/or government. Persons not in full position of their mental faculties are considered the property of other people, be it their legal guardians or the state. Prisoners are also in many ways the property of the penal system- their freedom of movement is greatly restricted and their ability to choose their actions is to a degree rather limited. Whether this is for punishment, for their own safety, or for the safety of society is, so far as this argument is concerned, a moot point. They are for all intents and purposes human property. If you’d like to take this even further, you could argue that everyone who is a citizen of any country is in many ways the property of that country, bound to abide by the rules that government sets forth, and subject to the decisions and powers of that gov’t. Go even further, and all property is theft. 
But, as I said above, there are still rules about what you can and cannot do with your property. (Check out the US Constitution if you’re confused about what I mean here.) So why the indignation when the gov’t says you aren’t allowed to abuse animals just because you own them?