A valid Catholic marriage results from four elements: (1) the spouses are free to marry; (2) they freely exchange their consent; (3) in consenting to marry, they have the intention to marry for life, to be faithful to one another and be open to children; and (4) their consent is given in the presence of two witnesses and before a properly authorized Church minister. Exceptions to the last requirement must be approved by church authority.
Protestant marriages require approval from a Catholic authority to be valid. It is my understanding that converts to Catholicism can easily have their protestant marriage blessed to make it a valid marriage. But normally, a Catholic marriage is a liturgical sacrament (celebrated during Mass). A Protestant marriage is never performed during a Catholic Mass.
The only question I do have is that, in the event where in regards to number 3, as long as there is the intention to marry for life as well as to be faithful, what about those who are beyond the ability to have children, by age or medical reason? I would think the most likely thing is to express fruitfulness via a life of doing good.