That is too simplistic. 
Marriage between Christians is sacramental the spouses are joined together by God. Natural marriages, those between non-Christians or between a non-Christian and a Christian are not joined together by God.
Nevertheless, the Church allows only the remarriage of the Christian spouse if the non-Christian spouse wishes to divorce. 
This is basic, there are more conditions than this which would require much more space and time to go through. These are available on the internet if you care to Google the topic.