Craig v. Boren was a supreme court case dealing with sex discrimination. There've been a lot of cases since dealing with sex discrimination-- and then later, talk about gender discrimination.
And that, I think, is a very interesting distinction to make.
My biology and psychology background shows some here, but sex and gender are two very different things—and neither of them is a binary. It’s easy for us to assume sex is a binary of male/female, but that ignores a number of intersex individuals. One of them most interesting examples to me has always been individuals with 5-alpha-reductase deficiency, so common in one area of the Dominican Republic that the condition is commonly referred to as Guevedoche there. If you've read Middlesex, it's the condition the lead character has. Individuals with 5-ARD are genetically, chromosomally male—but appear as female until puberty. Compare to individuals with Androgen Insensitivity Syndrome-- genetically male, yet often with a typically female appearance for their entire lives.
What's really nifty is that people with both conditions may develop a female gender identity—and those with 5-ARD may either continue to be female gendered, or become male gendered, when puberty hits. Even more difficult is the fact that other forms of intersexuality have chromosome configurations beyond XX and XY, showing that sex itself is certainly not a binary. Gender is an even broader spectrum. While people predominantly identify with the gender that matches their genitalia, you can also find people who identify as genderless while others are genderqueer, feeling they encompass more than one gender or that their gender changes with time.
In the choice between using the label sex discrimination and the label gender discrimination, I prefer the use of gender discrimination. Gender discrimination covers a wider ground, taking into account not only the male/female divide, but also transmen and transwomen, who might not be protected if it is merely sex discrimination. Protecting individuals regardless of gender—and indeed, regardless of how we perform gender—is paramount, and I believe that it encompasses traditional sex discrimination protections, while sex discrimination captures only a very heteronormative slice of things that can fall under gender discrimination.
It's so easy to think of things through the hetero, gender=sex mindframe. And really, it's how we've been thinking of it for... almost the entirety of America's existence. But that's got to stop sometime.
That time should be now-- both in our legal parlance, and our everyday lives. Sex and gender aren't the same thing, and treating them as if they are diminishes everyone whose gender doesn't tidily line up with their sex.