A marriage between two Protestants by a Protestant minister is assumed to be valid and sacramental. Marriage between pagans is valid too, although not sacramental. But it's invalid when a baptized Catholic gets married to a Protestant in a Protestant church without dispensation from form, because if you are a Catholic you are required to follow the Catholic Rite of Matrimony.
Very simple principle here. If you are a Catholic you are bound to follow ecclesiastical law. If you are Orthodox, Protestant, pagan, or Jewish you are still bound to follow the natural law, but you are not bound by ecclesiastical law.
I can think of one place here recently (a comment made by an individual who i the past has presented himself as being 'Catholic' priest, iirc) where I assume the individual's own understanding RCC canon law was invoked regarding past marriages of Donald Trump --- prior to himself marrying Melania.
I'll send the link by freepmail (that way, dragging dispute from there, potentially to this thread ---can hopefully be avoided).