Marriage is partly about children but the important thing to note is it's not a pre-requisite.Do you think if marriage did not usually lead to children or never led to children it would even ever have been invented? To what purpose? Marriage was invented in case you had children not so you could. Its all about inheritance and property rights.
Otherwise you can amend the law and demand a fertility test every year for every married couple and if they don't produce children the marriage should be terminated by the state.