People who wield privilege seek to protect that privilege by restricting those who have access to it. In doing so, they create "one drop" rules.
There is privilege in being white. If you have a drop of black blood, you're black. And they enforce their privilege through racism.
There is privilege in being upper class/upper caste. If you weren't born upper class, you can never ascend into that rank. And they enforce their privilege through classism.
There is privilege in being straight. If you do one gay act, you're gay. And they enforce their privilege through homophobia.
You get rid of one drop rules when you take away the privilege. When you have no privilege, there's nothing to protect.