Chiming in late here.....
Although I agree that Mary needs to get over it and leave the kid with a sitter if she wants to participate in some activities, I also don't see any indication that people have made any effort to do things to include her and her family. If that just isn't what the groups wants, then I think it is only fair to tell her that the group has no interest in family friendly activities.
However, I personally think that what Claire said is an absolutely SLAP in the face to Mary. It seems that everyone is willing to say, well...she was rude but Mary deserved it. I thought that rudeness because of rudeness what NEVER ok. Personally, I wouldn't care to even be friends with someone who said I chose to 'pop out a brat.' So it seems like the group would get what they want anyway - to get rid of Mary and her family. I don't think that anyone handled this well, but it seems that no one has even been willing to talk to Mary about the problem, just talk horribly about her. Yes, she has been PA and NO, I don't think her requests are reasonable at all. However, even PA people who are my friends, I try to work with them occassionally. At least then I can say I tried. And if it doesn't work out, I talk to them and tell them why.
Regarding the bolded, I don't think anyone is saying that what Claire said is ok. Everyone agrees that Claire went over the top with her choice of words and that she was incredibly rude.
But the point is that Mary is asking everyone to never ever do anything as a group other than meet at her and Mark's apartment and speak in hushed tones. To be honest, if I was a member of a group and one of the other members decided that we should one specific activity every single time we got together, even if that particular activity was the funnest thing ever, I'd object. The fun of getting together with friends is to do a variety of activities, otherwise it gets old. And on top of it, this group has acquiesced three times now, doing something they had not only had no interest in doing, but that made them completely uncomfortable for an entire evening, for the sake of this friendship. Instead of appreciating that and meeting the group half way, Mark and Mary now expect the group to do this every single time they get together. And when they don't, Mary posts snarky PA comments on the website.
Again, I don't condone or excuse Claire's comments in any way. She was completely out of line. But the original problem still exists. Mark and Mary are at fault here.
As a side note, when my kids were young, I had a group of friends who also had kids all around the same age range. We all got together fairly regularly but we did not do the same thing every time. We went out to a kid friendly restaurant or we got together at someone's house and played games or watched a movie, or we did a day activity and went to the park, etc. And yes, we did these kinds of things regardless of the fact that among us, we had kids ranging in age from infant to elementary school age.