Have you ever worried that you might be fired if you gave any inclination that you might be straight? You can be…with impunity…in any state which does not give equal employment protection based on sexual orientation.
It’s true that it rarely happens that someone would be fired for being straight, but it has happened.
To my knowledge, nobody has every been fired for being cisgender (i.e non-transgender). But there is no reason, in most states, why it couldn’t happen.
The examples of people being fired for being transgender are too numerous to include.
That’s the thing about laws protecting gender-identity or sexual orientation. They are for the protection of everybody, not just GLBT people.