Marriage as an institution is about 4500 years old. The tax advantages and other advantages conferred on married people are much more recent.

So this was far more about withholding something from a group than it was of granting something to that group.

Either way, on the total age of humanity that is an eyeblink and fortunately we are not above progressing. At least, not all of us.

On the total age of civilization it's pretty close to 100 percent. Shrug. Just be ready for the regression to the mean.