In the last few months I've found out I'm officially barren. (Good, didn't want kids anyway.)
Per gbaji, since the historical reason for marriage was or what procreation, my marriage ought to be worthless in the eyes of the law.
There are other economic benefits besides tax credits that are given to married units, not because the married couple in question may ultimately have children, but because the behavior of married people tends to shift a little. For example, the car insurance break that married men under thirty get. They give it to married men regardless of whether they've had kids or not, because once married, men tend to drive a little safer. Not just in anticipation of future children that they have, but also because they realize if they get themselves killed, they have a wife they're going to leave behind and she may or may not be in a position to provide for herself.
Ultimately, I think the tax breaks and other economic benefits given to married couples are not benefits to entice them to have kids, or to encourage them to get married because they procreate. It's because couples in long term relationships tend to be more stable and for the most part tend to be more model citizens. That has historically been the cast for the last three centuries, and the insurance actuaries bear it out to this day.
(Apologies for typos, I got pretty drunk at the Human Rights festival at the brewery tonight. )