hc4361, // Why do I need an annulment if I'm not Catholic?
If you're not Catholic, but plan to marry a Catholic, you might be asked to go through the annulment process. This seems odd to most non-Catholics because neither person from the first union is Catholic. Therefore, why should the Catholic Church investigate this marriage?
The Catholic Church presumes the validity of any marriage between two people who are free to marry at the time of their wedding. (They must have no previous marriages.) Basically, if the non-Catholic religious community of either spouse recognized the marriage, so does the Catholic Church. Since marriage, as God created it, is permanent, then the Catholic Church must also investigate these marriages. Because the non-Catholic wishes to marry a Catholic, the Church's law applies to the proposed marriage, since canon law still binds the Catholic whom the non-Catholic wishes to marry.
In short, the Catholic Church believes her teachings concerning the essence and the properties of marriage bind all people, regardless of whether they are Catholic, as part of God's natural law.//
http://www.catholicnewsagency.com/resources/life-and-family/marriage/catholic-marriage-and-annulments/