Is it a sin for a Catholic to attend weddings in Protestant churches?

The Catholic Answers

The law of the Catholic Church forbids participation in a religious service that is not Catholic because it is an implied repudiation of the faith which a Catholic professes to be the only true faith. It is good for non-Catholics to realize this so that, knowing that Catholics must refuse, they will not ask them to assist at the religious ceremony itself and then be offended as if refusal were due to lack of friendship.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty