I would never refuse to attend a wedding because I “dislike the venue”. That is shallow. That is not what my friend did. She dislikes Christianity and she has her reasons.
My view about marriage is the one taught by the Catholic Church and my friend knows it. I don’t demand that anyone share it, but I do insist that I be allowed to act according to my conscience. I expect no more nor less for anyone else, including my friend, whom I do not judge.
To cut off a good friend for acting in accordance with her conscience is sad, and, I think, immature. Of course if I were unkind about it, that would also be wrong.
This is just one of drawbacks of living in a pluralistic society, I guess. And in the end, people have the right to choose to be friends with whomever they wish, and to end those friendships even for sad and silly reasons. I’ve ended enough friendships for sad, silly, and proud reasons to regret it and to advise anyone else from making the same mistake.
Of course, this is all a moot point because I doubt very much that my friend will ever try to get married or have a family, but you are putting very ugly words in my mouth if you think I would regard her family as “a grotesque sham”. I don’t look at people that way, however I regard their decisions.