I just recently went through this exact same thing. One of my closest friends is a couple of years older than me (32 now) and we've known each other since 2014. She had never wanted kids and it was one of the things that gave us the closest bond to begin with, especially as the only 2 CF women in our office. Last year in Feb she let me know she was pregnant and she admitted to me that she didn't want to tell me because she didn't want me to 'hate her'. I obviously don't hate her, that's a little much.
But's it's definitely true that everything has changed and it will never be the way it was before her baby, and there is some silly feeling of betrayal under it all that made me grieve what our friendship was for a few months. She was the last of my CF friends so now I'm the lone wolf once more. More than anything I miss having someone to banter with about the joys of not having kids, which would obviously be inappropriate of me to share with her now lol. It's one of the main reasons I sought out this forum because I now have no one in my life of a like mind and found without an outlet I was becoming rather bitter at the whole subject.
If I wasn't as close to her as I am, the 'fizzle' would have been immediate after the birth last September, because that's how it worked out in other friendships of mine. I'm very socially awkward as it is and don't like to hang out with people or go to social outings even if I'm really close with people, so once a baby is born I just casually fade into the background and the reality is, they probably don't even notice until a few years later that I'm gone. It's also really hard to feign excitement and happiness for someone when that doesn't match your ideals at all; most of the people in this forum would see that as a death sentence so how do you act happy? Every time it happens to me I literally want to say the words 'Congratulations on kissing your life goodbye!', but I'm a nice person and keep those words on the inside lol.
I don't think it makes us a 'shitty friend'. Our friends know who we are, that this is what we want for our lives, and that we aren't changing our minds, so I like to believe they understand when we take a pretty big step back. And them knowing we don't want to be around a crying baby all the time or want to go places that babies aren't acceptable is probably one of the reasons their contact falls off as well. They're just two completely different cliques. You have theirs where they want to feel understood and have their self-made struggles acknowledged and be able to compare their experiences with other people going through the same thing. And you have ours where we want to completely ignore that world altogether and live a freer lifestyle not having to answer to anyone. They just don't mesh, it's like being Christian and Buddha. You'll find those friends will probably come to you with the stuff they could never take to the other mommies because of the judgment they would face, like how they hate having to be around other people's kids or think that Karen's parenting techniques are wrong. The ones who will babble on about nothing but themselves and their kids and don't let you get a word in edgeways or change the subject are the ones that have fallen so far off the path that you're probably better off moving on anyway, because nothing will ever be as important as themselves and their kids.