Question: Are Catholics allowed to see marriages in Protestant Church? "R.E.T."
Answer.—It is well-known that Catholics have always been forbidden to take part in any form of heretical or dissident public worship, either in a Church or elsewhere. The marriage service must be accounted a public religious rite and so Catholics are not allowed to assist at it in a Protestant Church or before a Protestant minister.
The rule of the Canon law (1258) is this: "It is unlawful for the faithful to assist in any active manner or to take part in the sacred services by non-Catholic.
"Merely passive or material presence may be tolerated by way of civil deference or for the purpose of showing respect to persons, at the funerals of non-catholics, at their marriages and similar solemnities, provided there is danger neither of perversion, nor scandal."
Under this rule, theologians generally condemn the assistance of Catholics as official witnesses, i.e: as bestman or bridesmaid at such marriages, as part of the official witnesses does seem to involve active co-operation in the religious ceremony. It must not be forgotten that, in practice every case should be submitted to the judgment of the parish priest or of the bishop.
Scandal varies very much with countries and sometimes with localities and what would be a cause of great scandal in some places might be quite harmless in others. The Faithful must always bear in mind that 'he who courts the danger shall perish in it " and that "prevention is better than cure."
- Malaya Catholic Leader,February 9th, 1935 (1935.pdf pp58)