An unnamed person told one of my friends (who will also remain unnamed) recently that they didn't think people should get married until they wanted to have kids, because that is the whole point of marriage. I strongly disagree! If you take that approach, you are basically agreeing with all the non-Christians who view marriage as simply a way to legitimize their offspring.
Also, the reason God gave for instituting marriage was not "to have kids". Rather, God created the first marriage because "It is not good that man should be alone" (Genesis 2:18a). He didn't create marriage because he wanted grandbabies, he created marriage because he didn't want people to be lonely! Of course, he did tell people to "be fruitful and multiply" (Genesis 9:7a), but he didn't list that as the main reason for marriage. It's more like a happy by-product. Besides, what about Paul's declaration that "it is better to marry than to burn with passion" (I Corinthians 7:9b)? It seems to me that God created marriage as a way to allow people to be happy and contented with their lives, instead of having to be lonely and frustrated. Children are a possible result of this state, not the point of it.