When “sex” was first included as a protected class in the Civil Rights Act of 1964, it meant that women couldn’t be excluded from employment opportunities just because they weren’t men. Over the years, a more nuanced view of gender (the less distracting term borrowed from grammar) has come to include the idea that people shouldn’t be penalized at work because they don’t exhibit stereotypical traits of their gender. A woman who acts like a man, or vice versa, should have the same opportunities as a traditionally feminine woman – and they should both have the same opportunities as a man.
In the last few years, this protected class has expanded again to include gender-nonconforming people, and transgender individuals are starting to be protected from workplace discrimination. Just recently this was further codified when President Obama signed an Executive Order adding gender identity to the list of protected categories for federal contractors.
Gender orientation is still not federally protected under “gender” for all employers, but it is protected in many states and President Obama included it in his recent executive order . In Montana, marital status is protected, so you can make a case that being in a same-sex marriage, while unrecognized in the state, is not a valid reason to fire someone. Given the trend in legal decisions and executive orders, it is safer to treat gender orientation as one of the protected classes.