As the article points out, the marriage of a Catholic and a non-Christian is an actual marriage, accepted by the Church (if the proper forms are followed), but is not a sacramental marriage, because non-Christians can receive no sacraments other than baptism. Such a marriage is a natural marriage rather than a sacramental marriage, and thus, is not valid in the way we generally use that word in the Church.
As the article further points out, the Catholic party does nothing wrong, is not in sin, is still a Catholic in good standing, even if contracting marriage with a non-Catholic. But it isn't a valid sacramental marriage.
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it is still a valid marriage, not a sacrament for the non-Catholic side.