Millions is a bit much. The Native American population simply wasn't that large and the percentage of "two-spirit" people was not significantly larger than it is currently. It should also be noted that not every tribe considered this to be sacred or performed such ceremonies. Native Americans were people, with all the foibles of people. They fought wars with each other, hated the "outsider" they had policies that were good and bad. The idea that because one tribe thought this way they all did is a fiction.