i feel marriage is not only about having children there are several other factors that constitute it. if childbearing was the main purpose, then marriages without kids would be considered failures!!! but that is not true. many couples stay happy together even without children i can testify to that. the foundation of marriage should be companionship, love, support and partnership between two mature people. children are a blessing when they come but they should not be the only reason to marry or stay married. the truth is that some people are not able to have children and others choose not to. that does not make their marriage less meaningful.
I think there are two school of thoughts on the purpose of marriage depending of what you believe in, the first is the Christian narrative which is marrying for the important purpose of raising God's children and the other benefits are added attachments whilst the social view is the one you have started, for companionship, growth and a life long friend.
From the Christian view, marriage with out childbirth doesn't mean the marriage isn't valid, there are still other options like adoption or other medical alternatives, the underlying statement is raising God's children.