Here is another answer to this question from a place called Radio Replies which is a reliable Catholic site for solid answers. I bolded some portions to help you focus.
1089. Is it a sin for a Catholic to attend weddings in Protestant churches?
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.
1091. Why is the Catholic Church so severe in her lata in this matter?
For very good reasons. Firstly, loyalty to Christ forbids our sanctioning in any way a false form of religion, and Protestantism is a corruption of Christ’s religion. If one may attend any religious services, irrespective of creed, then a Christian could assist at pagan rites. There must be a limit somewhere, and the Catholic Church says that those limits exclude any false form of religion, even though it be an adulterated form of Christianity. The presence of a Catholic at Protestant services is a silent approval of the error that one religion is as good as another. St. Paul says, “A man that is a heretic avoid.” Tit. III., 10. St. John says, “If any man come to you and bring not this doctrine, receive him not into the house, nor say to him: ‘God speed you.’” 2 Jn. V., 10. The law of the Church, too, protects the faith of Catholics. If they attend Protestant services, there is always a danger that they will participate actively in a shamefaced way, and also a danger of their drifting into indifferentism and weakening in their own faith. Their presence, also,** can be a cause of scandal to other Catholics **]who may begin to think that it is right for them also to attend at non-Catholic Churches. Nor is such attendance a kindness to Protestants. The abstention of Catholics from their services is a lesson of the utmost importance to them. Our attendance would sanction to a certain extent their idea that their religion also is as good as our own. But our absence from their Churches gives them food for thought. An Anglican might say, “Well, I have seen Methodists, Presbyterians, Congregationalists, and people of many other religions at our services; but I have never yet seen a Catholic associated with us.” And the fact that the vast Catholic Church denies their claims has led many a man from the chaos of the different Protestant Churches to the true religion.