N=1, from someone who didn't get kids:
The reality is that when the interests of friends change, the friendship itself might not survive. That's ok. People change. And with that, I feel that the chance of a friendship surviving such a life-changing event is tied to how encompassing their parent identity becomes.
If their former interests disappear (the ones on which we built our friendship), and every conversation inevitably becomes about their kids, then I will take a step back.
For example: The other day my wife mentioned to me that her sister (i.e. my sister-in-law) refers to their parents as 'grandma & granddad' when talking to her in private.