I agree that in Matthew's Gospel it is not really an exception, because it is only for unlawful marriages.
Annulement is granted only when the marriage did not exist. For a marriage to exist there must be true consent. When consent is lacking, there is no marriage. I do agree that this law has been grossly misused as a loop hole to get out of marriage.
Confession of sins does not change the fact that marriage covenant is permanant. In fact if one is truely sorry for their sins, they would unwrong the wrong they have done. With God's help this is possible.