What about when you're past child bearing years, and can no longer 'procreate?' Isn't a married couple's motivation then only physical union and pleasure?
Look at Song of Solomon. Sex (within marriage) is NOT just for procreation. It IS for pleasure. Ronald Reagan got that right a long time ago.
Let's go one step further and ask what if a couple is well within child bearing years and for some reason or other are physically unable to have children?
The answer is the same: every act should be open to the possibility of procreation - there are no guarantees that a child will result from any specific act.
And again I would hope that a married couple past their childbearing years would have more than mere physicality motivating them. I would hope they have sex because it helps them maintain the emotional and spiritual bonds of intimacy that they have for one another - not just to obtain physical pleasure.
The intent should always be to express one's love for one's spouse on every level to the fullest extent one is able.