Animal rights are no more a matter of opinion than is any other moral matter. This question is logically and morally indistinguishable from asking whether the morality of human slavery is a matter of opinion. We have decided that slavery is morally reprehensible not as a matter of mere opinion, but because slavery treats humans exclusively as the resources of others and degrades humans to the status of things, thus depriving them of moral significance.
The notion that animal rights are a matter of opinion is directly related to the status of animals as human property; this question, like most others examined here, assumes the legitimacy of regarding animals as things that exist solely as means to human ends. Because we regard animals as our property, we believe that we have the right to value animals in the ways that we think appropriate. If, however, we are not morally justified in treating animals as our property, then whether we ought to eat meat or use animals in experiments or impose pain and suffering on them for sport or entertainment is no more a matter of opinion than is the moral status of human slavery.
Moreover, as long as animals are treated as property, then we will continue to think that what constitutes “humane” treatment for your animal property really is a matter of opinion because you get to decide how much your property is worth. Just as we have opinions about the value of other things that we own, we can have opinions about the value of our animal property. Although our valuation of our property may be too high or too low relative to its market value, this is not generally considered a moral question. So when Jane criticizes Simon because he beats his dog regularly in order to make sure that his dog is a vicious and effective guard dog, Simon is perfectly justified in responding to Jane that her valuation of his property is not a moral matter up for grabs, but a matter of his property rights.
On another level, this question relates to a subject discussed in the Introduction, the position that all morality is relative, a matter of convention or convenience or tradition, with no valid claim to objective truth. If this were the case, then the morality of genocide or human slavery or child molestation would be no more than matters of opinion. Although it is certainly true that moral propositions cannot be proved in the way that mathematical propositions can, this does not mean that “anything goes.” Some moral views are supported by better reasons than others, and some moral views have a better “fit” with other views that we hold. The view that we can treat animals as things simply because we are human and they are not is speciesism pure and simple. The view that we ought not to treat animals as things is consistent with our general notion that animals have morally significant interests. We do not treat any humans exclusively as the resources of others; we have abolished the institution of human property. We have seen that there is no morally sound reason to treat animals differently for purposes of the one right not to be treated as a thing, and that the animal rights position does not mean that we cannot prefer the human over the animal in situations of true emergency or conflict where we have not manufactured that conflict in the first place by violating the principle of equal consideration.