Sadly, I have been placed in an analogous situation twice. The advantage of having an old Irish priest with a lot of miles on him is that he gives excellent advice (as is given above as well). In both cases, he said that if questioned about my refusal, to simply tell the party that in the eyes of the Church, there is no wedding to speak of! So, for example, would someone attend a birthday party and bring a gift when the person being celebrated doesn't even show up?
The responses I got were incredible. Yes, some of the family was not happy. However, other good, solid Catholics pulled me aside at later events and told me they wish "they had the guts to take that stand." They hadn't for fear of being disliked.
Thanks for posting this NYer. This situation is going to grow more and more common. To consent by your participation or acceptance is giving horrible Witness to the Gospel. I am reminded of a friend--an Orthodox mother-- who told her son she would not attend his wedding if it was not held in a Church, and her son and his fiancee relented. That is tough love!
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It is indeed! A good mother, she has my respect and admiration.