I only find it difficult to be friends do with childless people if they can't comprehend that I don't have the free time (or money, or energy) to gallivant around as if I were also childless
While that may of course be true of this specific set of friends, I do think that statements like this perpetuate the myth that the childfree just gallivant about, while parents are the sole group who move into the realm of responsibility and lack of opportunity to let their hair down whenever it suits them. Which I think is unfair to both groups, creates an artificially them and us situation, and logically ends up in insulting, sexist and infantilising nonsense like the suggestion that Theresa May (an obvious gallivanter - not!) is less suited to being PM than a woman with children.
There's also, I think, immense cultural pressure to believe that becoming a parent fundamentally alters a woman, and to 'perform' that change, which leads sometimes, imo, to struggling new mothers thinking they are supposed to shed all vestiges of their previous self and plunge into a whole new set of 'mummy relationships', like it's a rule.
It's slightly like the fact that despite the fact that there is no evidence whatsoever after major studies that 'pregnancy brain' exists as an observable phenomenon, lots of women still engage in competitive 'baby brain' anecdotes, which are as much about a culturally-mandated desire to perform pregnancy as life changing as anything biological, I think. We get so many more media images of the pregnant woman as serene and self-contained and 'other' than we do of pregnant women being perfectly ordinary - it was why Olivia Coleman's character in The Night Manager (who was both running a failing mini-organisation on no resources, running a spy and skulking around saving people with a gun while heavily pregnant) was such a revelation. She was a warm, quietly heroic, endlessly capable, committed and admirable character who was implicitly (rather than the other cliche of the chilly female spy/cop who is unsuited to motherhood) but her pregnancy was entirely incidental to the storyline.
I recently said on one of the threads about deciding whether to have children that I felt having a child (late) hadn't fundamentally changed me at all, much as I adore my son, and a couple of posters got very hostile and said that only a deficient human being wouldn't be changed by having a child.
Which comes back to erecting an artificial barrier between parents and non-parents. By which I mean mothers and women who aren't mothers, because it's clear that society doesn't expect men to be altered anywhere near as much by parenthood, and in fact they are far less likely to be expected to become SAHPs or go part-time in the workplace etc.