There were some interesting articles in the New York Times recently about testing female athletes at the Beijing Olympics if they were "suspected" of being male. (article and op-ed)
I agree with those articles that verifying gender is needlessly intrusive and problematic and should not be done for athletes, but disagree when the articles veer from athletics and try to assert the broader Postgenderist position that sex itself is needless and obsolete, malleable and fluid, and should not be fixed or determined by anyone for any reason. What does it mean to "live as" a gender, if not an offensive embrace of stereotypes?
There is of course one area where sex matters: reproduction. People with intersexed conditions, XXY chromosomes, and fluid gender expression have always existed, but never has any person reproduced as both a man and a woman, never has anyone both fathered and mothered offspring. Every person has one sex which they are most likely able to reproduce as, and one sex (the other sex) with whom they are most likely able to reproduce with, and that never changes throughout a person's life. It might not match, and need not define, the legal or social or apparent sex by which a person lives or even believes themselves to be, but it usually does and always should (though its OK when it doesn't). But that "most-likely-to-conceive-as" sex is what matters when it comes to reproduction: people should only be allowed to conceive as the sex which they are most likely to succeed as. After we enact the egg and sperm law, labs would make that determination if they are hired to facilitate a conception. Obviously merely living as or legally being the other sex cannot be a way around the egg and sperm law, or the law will not shut the door on genetic modification. But that lab-determination would be private, the public wouldn't know why the couple was unable to conceive.
On the other hand, if a couple is publicly, legally, a same-sex couple, but both-sexed by their most-likely-to-conceive-as sex, they would be publicly prohibited from attempting to conceive, even though their genes were in fact complementary. A lab would have to turn same-sex couples away at the door after looking at their legal sex, they wouldn't even get to the stage where genetic imprinting matters. If they were somehow able to do it naturally even though presenting as a same-sex couple, that should be cause to correct their legal sex (though not their gender expression).
To sum up, these articles are wrong that sex is fluid and cannot be defined. A woman is someone who would most likely be able to conceive with a man, a man is someone who would be most likely able to conceive with a woman. People should only be allowed to conceive as the sex which they are most likely to succeed as, and should be assigned that sex at birth, and their fertility and sexual identity should be protected as they grow up so that they are able to marry and procreate.