Annie, the Pauline privilege concerns only marriages contracted between non-Christians or between a Christian and a non- Christian. To this day such marriages are considered to be natural marriages and not sacramental marriages; the couples not being joined together by God therefore divorce is possible.
There are strict rules on this. Natural marriages cannot be considered dissolved on the say-so of one spouse, both have to agree to the dissolution. In the situation of a baptised Christian being married to an unbeliever, the baptised Christian can only remarry (in favour of the faith) another Christian.