Why would a liberal object to Aristotle's understanding of political association?

The major difficulty in answering this question has to do with defining liberal. The term is thrown around somewhat at random in U.S. politics, and means something quite different than classical liberalism or neoliberalism, which in most other western countries are considered similar to the philosophies espoused by many "conservative" or Tory parties. For example, neoliberalism advocates free market economies while, for example, many people called "liberals" in the United States advocate strong government regulation and intervention in the economy.

There is one aspect of Aristotle's theory of political association that most modern people, including liberals of all stripes and almost anyone not a member of a neofascist political movement, would find objectionable. This has to do with Aristotle's theory of "natural slavery."

Aristotle believes that some people are "natural" masters and others are "natural" slaves. The former are have a high degree of innate intelligence and thus are most fitted to rulership. Natural slaves possess brute strength but lack higher order reasoning skills. According to Aristotle, certain races, such as the "barbarians" of northern Europe, due to their cold, sluggish natures are only suited to be slaves. Political association brings together slaves, who do brute physical labor in exchange for being guided by the reason of their masters, who develop complex forms of social organization, medicine, architecture, and other elements of society that require intelligence.

Aristotle not only believed that certain races were naturally inferior, but he also believed that women were naturally inferior to men. He objected strongly to democracy, feeling that societies should be run by a small group of qualified men. Most liberals (and, in fact, most people in modern western societies across the political spectrum) would find the sort of racism and misogyny repugnant.