Stentor analogizes the argument that humans and only humans should have rights to two exceptionally frustrating arguments for sexism and heteronormativity:
A1. Most heterosexual couples are able to produce children.
A2. Only those couples which can produce children should be allowed to marry.
A3. Therefore all heterosexual couples should be allowed to marry.
B1. Most women have less upper body strength than men.
B2. Only people with great upper body strength should be allowed to be firefighters.
B3. Therefore all men, but no women, should be allowed to be firefighters.
C1. Most humans (and few if any animals) are capable of “reason.”
C2. Only things that are capable of “reason” have rights.
C3. Therefore all, and only, humans have rights.
The point is that it’s inconsistent for liberals to support argument C while opposing A and B. But in fact, off the top of my head, I can think of two big discrepancies, one between A and C, and one between B and C. Argument A’s second premise is very weak; when questioned, its supporters can rarely come up with a justification better than “It’s what marriage is for.”
In addition, even assuming A2, it’s very easy to come up with a system that only gives fertile couples the right to marry: make marriage contingent on childbirth. It’s fairly easy to measure upper-body strength too, but not capacity to reason. There’s probably an objective standard for what a rational being is, but we don’t know it yet. We have a multiplicity of standards, the crudest easy approximation for which is “all born humans with functioning cortexes, and only them.”
The other big discrepancy is the degree of difference. When liberals argue for gender equality, they typically don’t attempt categorical refutations of arguments like B; instead, they point out that the differences between the sexes are small, and vastly outweighed by intra-sex differences. This is especially true in fields based on intellectual performance, where innate gender differences are trivial to nonexistent.
In contrast, species differences are enormous. The best-trained apes never master more than a few hundred words and no grammar at all, and are incapable of speaking. The most severely retarded human can know much more than that given enough effort. Even granting that a 30-year-old chimp may be more capable of reason than a 1-year-old, the overlap between chimps and humans is small enough, and the inconvenience of treating all chimps as persons or of establishing criteria for chimp personhood is large enough, for it to be moral to base rights on humanness.
Incidentally, it makes some sense to establish a middle category of beings that aren’t considered rational but are close enough that they should have limited rights: dolphins, great apes, very late-term human fetuses, octopuses, and so on. Their rights should under no circumstance be allowed to conflict with the well-being of humans – including the benefits of scientific research – but when their rights are sacrificed, it makes sense to do it in the way causing the least amount of pain.