Why is this so hard?

Human beings (a.k.a. the current Homo Sapien variety) have been walking the earth for about 200,000 years give or take a day. And in all that time we seem to just be the slowest, most stubborn species imaginable when it comes to social evolution. Here we are 200 millenia removed from the first of us walking the face of this planet and there are still a large group of us in the civilized world that feel that some of us aren't equal to others, at least as far as rights go.

Why do we find it so hard to accept these seemingly basic concepts? I know Conservatives and their supporters rationalize this by saying "Oh we can let them have civil unions." civil unions are not marriage, they're civil unions. If civil unions were marriage they wouldn't be called civil unions they'd be called marriage.