I was delighted to see 3 million people at Mass on a Rio beach with the Pope. However, this is in contrast to the Church's rules that a marriage must be celebrated in a church. What do I tell my nieces who want to have weddings outside?
All the sacraments, as a general rule, should be celebrated in a sacred space. Therefore, a dedicated parish church or oratory is almost always the proper place for the celebration of any sacrament.
But, as is almost always the case with general norms, there are exceptions. For example, in danger of death, baptisms are sometimes celebrated in the hospital. On account of urgency, or as the result of a pastoral moment, confessions are sometimes celebrated in settings other than the church.
In the example you cite, no Church building exists to accommodate the three million who assembled in Rio. The use of the beach was actually a backup plan that had to be implemented when the large open field that had been designated was rendered soggy by pouring rains.
Hence, for urgent pastoral reasons, many general norms can be adapted where necessary.
Regarding weddings, certain permissions can be obtained for weddings to be celebrated outside of the sacred space. However, the reasons ought to be serious, not just because it would be more convenient or pleasing to someone in the wedding party.
While permissions are sometimes granted, most dioceses resist granting these permissions too easily. Of all the Sacraments, the celebration of Holy Matrimony tends to be most influenced by secular trends. And many of these trends take the focus off Christ and the actual Sacrament that is being conferred. The emphasis too easily falls on dresses, flowers, food, and other social aspects. Moving weddings to beaches, backyards, reception halls and other such places, shifts the focus even further away from the Sacrament itself. It also tends to open the doors even further to certain passing trends, many of which are questionable, even frivolous or scandalous.
Hence, the celebration of the Sacraments ought generally to take place in the parish church. For serious pastoral reasons, such as stated above, exceptions can be made. But weddings seldom present pastoral conundrums significant enough to warrant the movement of the Sacrament outside the church and, more problematically, shift the focus even further from where it should be.
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