brb3,
… It would be nonsensical to say to that Protestant/Protestant church marriages are valid … while Catholic/Protestant marriages in the same Protestant church are invalid.
The Catholic Church has defined natural marriage (one or none baptized) and sacramental marriage (both baptized). For at least one Catholic, to be sacramental it has to be with approval of the Church and between baptized man and woman. For other churches the Catholic canonical form is not required. And interestingly when a Catholic and Orthodox marry in the Orthodox Church, it is valid.
From Archdiocese of Los Angeles
archdiocese.la/prayer/sacraments/tribunal/ministers/form.html
If a Latin Catholic married an Orthodox Christian in an Orthodox Church on or after March 25, 1967, a Lack of Form case does not apply (Crescens matrimoniorum, 22-ii-1967).
If an Eastern Catholic (except a Ukrainian Catholic) married an Orthodox Christian in an Orthodox Church on or after January 21, 1965, a Lack of Form case does not apply (Vatican Council II, Orientalium Ecclesiarum, 21-xi-1964). For Ukrainian Catholics, the applicable date is April 7, 1965.
If the marriage took place on or after October 1, 1970, it is necessary to clarify whether a “dispensation from canonical form” was granted for the marriage (Matrimonia mixta, 31-iii-1970, and canon 1127). If it was, the case becomes a Formal Trial rather than a Lack of Form case. The current of Form petition asks this question.
If the marriage took place on or after November 27, 1983, it is also necessary to ask if the Catholic party to the union had “formally” left the Catholic Church (e.g., enrolled in another Christian denomination or made a formal profession renouncing the faith; canon 1117). If something like this took place, a Lack of Form case might still be possible, but the circumstances surrounding the person’s decision to leave the Catholic Church would have to be assessed by the Office for Canonical Services.
Note: Dec 21, 2010 no more consideration of formal defection from the Catholic faith.