Pope Pius XI explains in his encyclical Casti Connubii:
They, therefore, who rashly and heedlessly contract mixed marriages, from which the maternal love and providence of the Church dissuades her children for very sound reasons, fail conspicuously in this respect, sometimes with danger to their eternal salvation. This attitude of the Church to mixed marriages appears in many of her documents, all of which are summed up in the Code of Canon Law: "Everywhere and with the greatest strictness the Church forbids marriages between baptized persons, one of whom is a Catholic and the other a member of a schismatical or heretical sect; and if there is, add to this, the danger of the falling away of the Catholic party and the perversion of the children, such a marriage is forbidden also by the divine law."[62] If the Church occasionally on account of circumstances does not refuse to grant a dispensation from these strict laws (provided that the divine law remains intact and the dangers above mentioned are provided against by suitable safeguards), it is unlikely that the Catholic party will not suffer some detriment from such a marriage.Whence it comes about not unfrequently, as experience shows, that deplorable defections from religion occur among the offspring, or at least a headlong descent into that religious indifference which is closely allied to impiety. There is this also to be considered that in these mixed marriages it becomes much more difficult to imitate by a lively conformity of spirit the mystery of which We have spoken, namely that close union between Christ and His Church.
Assuredly, also, will there be wanting that close union of spirit which as it is the sign and mark of the Church of Christ, so also should be the sign of Christian wedlock, its glory and adornment. For, where there exists diversity of mind, truth and feeling, the bond of union of mind and heart is wont to be broken, or at least weakened. From this comes the danger lest the love of man and wife grow cold and the peace and happiness of family life, resting as it does on the union of hearts, be destroyed.
My marriage was a mixed marriage when we got married. Why is that bad?
Well, you are taking that in the wrong sense. I was saying, "worse yet, when you consider the number of mixed Marriages, the number of Catholics marrying inside the Church is even lower, because non-Catholic marriage partners mean even fewer Catholics marrying within the Church."
However, in general, mixed Marriages do not work out, and Catholic Tradition discourages and forbids them. For every sucess story there are more failures, because of religious friction. And frequently, when they do "succeed", the parents and children drop out of Church to create peace in the family.
The ideal for two people of disparate cult who are in love would be for one of them to convert prior to the marriage, especially if one of them is not Baptised.