I think part of the difficulty with periods is that for kids (and many male adults) the idea of blood/pain/illness is inextricably linked and therefore potentially a disturbing concept.
You all lie to everyone else all the time. You must do, that's how social conventions are maintined. The average person lies 50 times a day(actually I made that up, can't remember the statisic but it seemed ALOT.) Why are you so hung up about lying to your children? You do it anyway, either by ommision, or to protect them, or boost their confidence, or make them happy, or because it's not the right time, or place, or it doesn't seem important...etc...etc...
And telling your child childbirth hurts 'a bit' is not lying by ommision, it's just plain lying. So as not to scare them.
And a good point was made earlier, what do you say if they ask if periods hurt? They do, for lots of people, and like hell for some.
I actually don't have a problem with telling them if you and they seem comfortable, but this never lieing stance I find bizarre and unsustainable.
I also agree that it's definetly cultural. but me and my kids are not German (or any other more liberal nationalaity) and we live here. I don't want mine to be the first in the class unecessarily burdende with knowledge.
Just play Star Wars, much more fun.