Do you have a target audience in mind? I think the explanations would differ depending on whether you are speaking to someone who is already in the Catholic Church, to someone who is Christian but not Catholic, or to someone who is not even a Christian.
In each case I think you have to address the authority role the Church hierarchy plays in the life of Christians. Someone who believes that “the Church” is just a name given to a group of people who have similar beliefs is not going to respect the authority of the Church to have a say about the validity or permanence (let along sacramentality) of marriage.