One striking difference between the Vatican II Preparatory Commission's schemas (https://isidore.co/calibre/#panel=book_details&book_id=4253) and the final Vatican II documents is what On chastity, marriage, the family, and virginity
(https://jakomonchak.files.wordpress.com/2012/09/on-the-sexual-order.pdf) has to say about mixed marriages:
Quote from: Vatican II Preparatory Commission document «De castitate et al.»21. Mixed Marriages Where a marriage between two Catholics can be contracted without extraordinary difficulties, the good of religion for the most part requires that Catholic men and women avoid so-called mixed marriages, especially with unbelievers. But the faithful also have the duty, in accordance with the dictates of prudence and the other virtues, to avoid marriage with those who are opposed to God or religion or with those who are Catholics in name but not in life. Although the Church, using her power, may permit mixed marriages, nevertheless the Catholic party, as divine law dictates, must in the conceded mixed marriage avoid dangers to the faith and indifferentism, must always carefully see to the Catholic education of the children, and lovingly and prudently try to bring the spouse to the Catholic truth. Pastors should take special care for those who are joined in a mixed marriage. The Sacred Synod knows that in some places mixed marriages cannot be avoided, but from the fact that this can happen in some places false principles or dangerous inducements should not be deduced.
... It also rebukes those who say, and indeed under the pretext of benefitting the Church, that mixed marriages are generally and in themselves to be fostered rather than tolerated. That position is also mistaken which maintains that a marriage can be declared invalid or dissolved solely because of a failure of love. ...
Quote from: fn 34Pius VII, Brief Etsi fraternitatis, October 8, 1803 (CIC Fontes, II, p. 718):
Quote from: Pius VII, Brief, «Etsi fraternitatis», October 8, 1803And the first of these is that the Catholic Church has always forbidden and rejected as illicit, pernicious and detestable the marriages of Catholics with heretics, as we could demonstrate from innumerable decrees of Councils and of Supreme Pontiffs.... And although in some areas because of difficulties of time and place, such marriages may be tolerated, this should be considered to be an equanimity which in no way implies approval or consent but mere patience, necessary but not voluntary, in order to avoid greater evils...;collate this with many other documents, especially those listed in the note to D 1499 and in the note to CIC c. 1060 (https://books.google.com/books?id=2XbtF6Y21LUC&pg=PA365). With regard to canons 10 and 31 of the Council of Laodicea (Mansi II, 565 and 570), it should be noted that they are to be interpreted in the light of the whole teaching of the Council, which did not even permit the faithful to pray with heretics and schismatics.
Vatican II threw this teaching out due to Vatican II's promotion of the heresy of ecumenism.
1917 CIC c. 1060 (https://books.google.com/books?id=2XbtF6Y21LUC&pg=PA365) says:
Quote from: Canon 1060Most severely does the Church prohibit everywhere that marriage be entered into by two baptized persons, one of whom is Catholic, and the other belonging to a heretical or schismatic sect; indeed, if there is a danger of perversion to the Catholic spouse and children, that marriage is forbidden even by divine law.
The 1983 Code on mixed marriages (http://www.vatican.va/archive/ENG1104/__P41.HTM) is much, much more lenient.