Could anyone provide details? I wonder whether it is an explicit "Being gay is grounds for firing" or rather just plain lack of protection from firing for being gay?
It doesn't miss it, that would imply an unintentional oversight. There might have been a time when it was just not something people thought of including (although the fact that homosexuality was often explicitly criminalized also suggests that was not the case), but by now there is no state where there has not been an effort to get such protections on the books, so it is, in every case where it is not explicitly protected, a deliberate exclusion.
It makes all those "protections" completely toothless.
In order to win a case, you would need to provide extensive documentation, probably collected via clandestine recordings, that would be able to convince a jury by preponderance of the evidence that you were fired for a protected reason, and not simply because the company no longer wanted to pay you for your work.
Besides that, one of the selling points of "at will" was that making it easier to fire people would make it easier to hire people. Anecdotally, I have not found this to be the case. Companies simply find it easier to discriminate based on things like race, gender, perceived sexual orientation, weight, religion (or lack thereof), disabilities, appearance, or age, because it is easier for them to deny that those are the factors that they consider in hiring and firing.
As Apple execs engaged in collusion with other companies to weaken workers as a class via an anti-poaching cartel, Tim Cook is hardly able to take a non-hypocritical position about sexuality discrimination in the workplace. When all workers are weakened, the ones that are most often discriminated against often suffer the most, because they are the marginal hires in more places.
Sexual orientation isn't a protected class under Federal law. (See: http://www.eeoc.gov/laws/types/ )
State Human Rights Law in some states expands to incorporate other protected classes, including sexual orientation and transgendered status.