I’m not married, but there are some observations that can be made and some potential complications that are evident simply by knowing the Faith and what it asks of a Catholic Parent.
I think it is an unwise idea for a Catholic to marry a Protestant. It’s not a sin, but a Catholic considering marriage must remember the duty to do everything within his or her moral power to rear the children totally and 100% Catholic. If you’re considering marriage, but not married yet, one such thing within your power is to marry a devout Catholic spouse who therefore, by default, will contribute to the children’s Catholic Faith. Consider that you are not quite doing 100% everything in your power to rear your children Catholic when you agree to marry a Spouse who is less inclined to help you, 100%, in that regard. Now, some aren’t guilty of this failure since they simply aren’t thinking of that implication when they choose to marry a non-Catholic spouse, but those aware of this implication may want to give it a second thought.
If you do decide to marry a Protestant, you as a Catholic owe it to your children (if it occurs to you–again, not every Catholic to not do this is guilty of deliberate omission) to make sure the Protestant potential spouse agrees to do everything within his/her power to help you raise the children Catholic, and does everything possible to avoid having the children abandon the Catholic Faith for his/her Protestant Faith. It is the duty of you the Catholic to establish this, not of the Protestant to volunteer it, and I believe that’s what the Canon Lawyer meant when she spoke to Lil_Cat, for instance: A Protestant is not willingly under the authority of the Catholic Church, and thus, unlike the Catholic, they only have the responsibility to raise the children Catholic if they embrace it willingly… Of course, if a Catholic is quite thorough in his/her plans to raise the children totally Catholic, he or she will want to secure such a promise before agreeing to marry a Protestant Spouse in the first place. Then, Canon Law notwithstanding, the Protestant is just as responsible as the Catholic by virtue of that promise–as both Protestants and Catholics can agree on the value of keeping one’s word, especially if the whole agreement to marry rested on such a promise.
I will say that, even if the Protestant Spouse makes this promise, it could be quite complicated…they cannot, if they are sincere Christians, lie to the children, nor can a sincere Catholic expect them to; this makes for awkward situations when the children ask the Protestant parent religious questions. Plus it could get very lonely for the Protestant to not be able to fully share his or her faith with his or her own children, even if he or she had thought it would be no problem. There are ways around these matters, but they do make things more complicated for a household hoping to do everything in both parents’ power to rear devout Catholic children. There are potential hardships for both parents in a mixed marriage that simply don’t exist if both parents share one Faith.
To the Protestants (married to Catholics) who are so angry at the Church for daring to hope that your children will be Catholic, totally so, instead of joining your own faith, consider that Catholics do not think of ourselves as a denomination. When I was Protestant, I didn’t care what denomination my future children might be part of as long as they believed in the bare necessities: The Bible. Now that I’m Catholic, The Bible, The Sacred Tradition, the Teachings of the Pope and Magisterium, and all those specifically Catholic Beliefs are the “bare necessities.” Deliberately allowing any children I might have to be instilled with even one value that contradicts those things would be the equivalent of a strict Bible believing Protestant marrying a more liberal “Christian” who believed sometimes the Bible could be disregarded (say, they thought abortion was okay), and not caring if the liberal spouse instilled such values into the children. If you’ll look at it that way (and I’m not trying to offend anyone; it’s honestly the best analogy I can find) then you’ll see why this matter is so important to Catholics. You may even agree to do more to help them raise the children Catholic if you come to appreciate the situation a Catholic parent is in.
For those Catholics already in such marriages who unfortunately didn’t take these things into account, realize that God expects you to remain faithful to your marriage, even if your Spouse fails to appreciate your duties as a Catholic parent…as an unmarried person, I humbly submit these suggestions: Just lovingly do everything presently in your power personally to rear the children Catholic, and pray that they will not be swayed away from the Faith. Try to get your Spouse to understand your duties and respect them, even if they don’t embrace your efforts themselves. Be respectful of your Spouse’s faith, never teaching your children to antagonize them and not doing so yourself, but at the same time be uncompromising in the way you rear your children as faithful Catholics. And again, pray, pray, pray.