I once was invited to a wedding where the man was Catholic and he married a Protestant outside the Catholic Church. Is this a valid marriage? I didn’t attend due to my husband having to work, but for future knowledge, would it have been wrong for me to attend? Would it have been a mortal or venial sin? I really never wanted to go, until I got an answer from someone who knew, so I was happy to not have gone because of this. Thank you for your response.
The marriage may have been valid. Under certain conditions, Canon Law allows a Catholic to marry a non-Catholic:
“Without express permission of the competent authority, a marriage is prohibited between two baptized persons of whom one is baptized in the Catholic Church or received into it after baptism and has not defected from it by a formal act and the other of whom is enrolled in a Church or ecclesial community not in full communion with the Catholic Church.
“The local ordinary can grant a permission of this kind if there is a just and reasonable cause. He is not to grant it unless the following conditions have been fulfilled:
"1/ the Catholic party is to declare that he or she is prepared to remove dangers of defecting from the faith and is to make a sincere promise to do all in his or her power so that all offspring are baptized and brought up in the Catholic Church;
"2/ the other party is to be informed at an appropriate time about the promises which the Catholic party is to make, in such a way that it is certain that he or she is truly aware of the promise and obligation of the Catholic party;
"3/ both parties are to be instructed about the purposes and essential properties of marriage which neither of the contracting parties is to exclude.” (Code of Canon Law 1124-1125)
Also, with proper permission, the marriage may be celebrated in another church:
“A marriage between Catholics or between a Catholic party and a non-Catholic baptized party is to be celebrated in a parish church. It can be celebrated in another church or oratory with the permission of the local ordinary or pastor… The local ordinary can permit a marriage to be celebrated in another suitable place.” (Code of Canon Law 1118)
Without the proper dispensations the marriage would have been considered invalid. For help in deciding whether to attend an invalid ceremony, I suggest reading Should I Attend?: cuf.org/nonmemb/attend.pdf