Marriage first became a legal issue when the Roman Empire feared their people were not producing enough children, particularly from known lines. This is when they decided to give a tax exemption to married couples and a tax hike to single people. It was a patriarchy and knowing who the father of the children was, was of great importance so that inheritances, including job titles, could be passed down. So if you look at marriage as a legal issue, or a traditional issue, yes procreation seems to be the vital element.
That being said gay and lesbian couples are more than capable of reproducing. They're not infertile and they have the same access to reproductive technology straight people do. There's even a new emerging technology that allows a fetus to be formed by manipulating two eggs, without sperm! (Sorry guys, the reverse is not true for you.) And of course heterosexual couples do not HAVE to have children to be married. It's an option, not a compulsion. Not everyone wants or can have kids, that has nothing to do with sexuality.
Marriage gets a bit trickier when you consider it a religious issue. A union under God can only be recognized if you believe your God is consenting of such a union, but this seems to be a deeply personal issue. Even if you worship the same God as your neighbor you may have completely differing views of what He's telling his followers.
I wrote an article The History of Modern Marriage that goes over all of this if anyone really wants to delve into the issue.