Interesting thread.
I'm childfree by choice (though sometimes wavering now that it's "crunch time" = 38. I certainly don't dislike kids and have never been able to say "never, ever") and I suppose I'm what people would call a high achiever (did very well at school and university and now run my own business).
There is the element of having more to lose, I suppose, if you have a well-paid career. I can't imagine how I'd be able to do my job after having only slept 2 hours, etc., and I wouldn't have the luxury of maternity leave because I'm self-employed. I have a very equal relationship with my DH but often observe that that balance goes out the window when children come along, also because of biological factors that don't tie in well with equality (childbirth, breastfeeding).
And after years of being able to fund nice holidays/meals out, etc., it's very hard to imagine having to give all of the things you enjoy up (at least if the things that make you happy are obsessive travel and dining out - from what I read on here, certainly not the most compatible with small children) for something that you have never experienced with benefits that are hard to grasp, at least for me.
I think high achievers often have (or have been socialised to have) very high expectations of themselves as well. To me, a baby would be yet another thing that I'd feel I'd have to do exceptionally well and that seems like a lot of pressure on top of keeping up my career, maintaining a good relationship with my DH and friends, staying fit and healthy. Especially if all of the things that I currently use to compensate for stress (exercise, excitement about travel) go out of the window, it just seems like a long hard slog for an uncertain outcome.