Even if a couple can't or chooses not to have children, it doesn't change the fact that their union under ideal conditions would produce children.
Under ideal conditions I'd be living in an Woody Guthrie tune called Big Rock Candy Mountain. Turns out that my knees aren't quite solid enough to be a hobo though. Who knew?
Having been raised in the Catholic tradition I can tell you that pregnacy and marriage have nothing to do with each other. Having been raised around farm animals I can tell you that preganacy and sex are only nominally related. Having had sex more than once I can tell you that there are other reasons for participating in what you would likely term "fornication" than creating children. Having been married for a decade and a half I can tell you that neither procreation or sex have a whole lot to do with any of that.
Marriage is an official agreement between two people to take a chance on each other. No more, no less. It has nothing to do with which part fits where or the creation of children.