On Fri, 15 Jul 2005, andie nachgeborenen concluded a summary of Posner's argument:
> I should mention that Posner is theoretically a sexual
> libertarian himself, who dispises anti-gay prejudice
> of any variety. The long and short of his theory is
> that true bisexuality exists but is likely to be very
> rare.
Posner's argument (at least as summarized by Justin, I haven't read the book) is an example of the "culture-as veneer" fallacy: "true sexuality" is a biological/genetic substrate, determined in the final analysis by evolution, and culturally learned ideals/values/identities linked to sexuality are just a thin facade that smooth the rough edges of sexual desire.
Bluntly, this is a fundamental misrepresentation of sexual identity in human societies. Whatever genetic or biological predispositions a person may have, they can only have a sexual identity if people in their society construct social groups and social identities that differentiate, reinforce, celebrate, and derogate specific sexual types.
For example: the assumption that sexual behavior reflects a stable, underlying, "true" sexual identity is in fact a historical and social accomplishment. Many societies do not have words/concepts that map to our idea of stable sexual categories at all (gay/straight/bi). Sure, people have all kinds of sexual activities, but they aren't expected to classify themselves as stable sexual types ("I'm straight", "I'm gay"), and so they just don't worry about coming up with what we think of as a stable sexual identity. (See Herdt, Third Sex, and Halpern, 100 years of homosexuality, for many examples.)
So when we talk about whether or not "homosexuality is adaptive", we're taking something that has in fact been socially created--stable sexual identities--and assuming that human beings at all times think about sexuality the way we do. It's presentism and ethnocentrism through and through!
So who is a "true" bisexual? A "true" lesbian? --A person who (a) lives in a society that includes those sexual categories, (b) accepts that sexual identity, and (c) is accepted by those around them as a member of the sexual identity group. In that sexual categories, practically speaking, are socially and historically constructed categories, the biological characteristics of the members of those categories are neither necessary nor sufficient conditions for inclusion in a specific sexual category group.
To anticipate one response, the above argument is not a claim that "genetics has no effect on human sexual preferences". The twin and adoption research is pretty clear: sexual preference is influenced by genetics (although "homosexuality" is far from Luke's analogy of genetically inherited diseases: the concordance rate for homosexuality in identical twins is about 50%, which means that I can be a genetic clone of a gay person and not be gay myself!). However, this genetic research is irrelevant: regardless of whether or not people have genetic predispositions to engage in sexual activities with certain types of partners, they can only "truly" be gay or straight if they live in a society that requires people to map sexual activity to stable sexual identities.
Miles