Eugene Volokh shows that none of the usual justifications for excluding gay men from the military extend to gay women. Yet as far as I can tell the set of people who want to exclude gay men but not gay women from service is empty.
Since I’m a rude liberal policy analyst rather than a polite libertarian law professor, I’ll draw the conclusion Eugene merely implies: the basis for DADT and its predecessor policies is bigotry. If you didn’t think gay people were icky, you wouldn’t want to keep them out of the service.