Three arguments

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.

As far as I can tell, these three arguments are essentially identical in their logical structure. And it seems that any liberal would recognize that arguments A and B are fallacious -- the ecological fallacy, to be precise (as well as disputing the truth of premise 2, but that's beside the point for now). Yet most of them (Kian's post is the example that motivated me to post this) would assert that argument C is sound. Indeed, argument C is at the heart of the most common rebuttal to the idea of animal rights.