Reason? Because i was not catholic and she could never bring her self up to be with someone who was not catholic too. Obvious implication was that non-catholics were, well, lower creed and not equally worthy. Needles to say, the room we were sitting in went silent. My jaw was on the floor. Those few seconds of awkward silence were broken down by my better half who re-directed the topic in another direction (or just asked if anyone wants another drink).
As far as I know neither the Catholic or Muslim religions ban people marrying someone of a different faith. Lots of rules about the children and they may recommend against it but its sure not banned.
Not wanting to marry a non-Catholic is more of a culture thing