The headline (Mixed marriages lead to fewer Catholic children) is catchy, but misleading.

Let's consider what happens using two groups of men and women.  Half of each group is Catholic, the rest are non-Catholic.  When two of the Catholics marry, 62% of the time the children will be Catholic.  But when two of the non-Catholics marry, the likely that their children will be Catholic would be nearly 0%.  So, on average, just 31% of all the children would be Catholic.

When two Catholics marry a non-Catholic rather each other, there are twice as many marriages with a Catholic spouse.  According to the Pew survey, when Catholics marry Protestants, 29% of all the children would be Catholic.  If we change Protestant to no particular faith, 32% of all the children from the two marriages would be Catholic.  So in the end, the total number of Catholic children would be roughly the same (and almost certainly within the margin of error) with or without mixed marriages.