Why, when a non-Catholic man and woman are married in a non-Catholic ceremony which is not presided by a Catholic priest, does an annulment have to be obtained in the event one becomes Catholic and wants to get married again?
Marriage is indissoluble by any human power. Jesus said, “What therefore God has joined together, let not man put asunder” (Mk 10:9).* The Catholic Church takes this seriously and, therefore, will not take part in a new marriage when she believes another valid marriage may already exist. This is true even if that marriage is between non-Catholics, married outside the Catholic Church – such marriages are recognized by the Church.
Civil divorce is often man’s attempt to put asunder what God has joined together and the Church knows that man does not have the power or authority to do this. The annulment process is simply the Church’s investigation into what looks like a marriage to determine whether a valid marriage really exists. If it does, the Church will not, indeed cannot, recognize another marriage. If, on the other hand, the Church finds that a valid marriage does not exist, then a new marriage, truly a first marriage (unless a valid previous marriage ended through death), may be celebrated.
*Note: The apostle Paul taught of a variance to this - in the case of a marriage between two non-baptized persons when one party later becomes a Christian: “if the unbelieving partner desires to separate, let it be so; in such a case the brother or sister is not bound. For God has called us to peace” (1 Cor 7:15). For more on this, see Divorce (in Moral Theology).