Yes, it is. Almost all world religions prohibit interreligious marriage or severely restrict it. They do this for several good reasons. First, it is necessary for the cohesion of the family. The old adage, “The family who prays together, stays together,” is more than just a nice platitude. When a family is united in faith, it is united at its very core. Interfaith families have to deal with this lack of cohesion which lies at the very foundation of the family. Second, and related to this, sad experience has shown that interreligious marriages fail at more than three times the rate of marriages where couples share the same faith. Third, experience has also shown that there is a real, grave danger that one or both parties will lose their faith altogether. All too often, an interreligious couple will avoid conflicts surrounding religion by ceasing all religious practice. Sometimes, for the sake of harmony, one partner will agree to convert to the faith of their spouse, even though they do not agree with it. In such situations, conflict is inevitable.
For these reasons, the Church prohibits interreligious marriage. That being said, the number of couples approaching the Church who are seeking an interfaith marriage has become increasingly common. The competent diocesan authority (i.e. the bishop or his delegate) may grant a dispensation from this prohibition, but each case is judged on its own merits and there are no guarantees that a dispensation will be given. In any event, if a couple is getting serious about their relationship, one’s faith should be the center of the conversation.