I don't think I ever heard the term 'child free' as a way of defining people who don't want children before I came onto MN. In RL, we speak of 'child free time', referring to times when parents are away from their children for some 'me time' or 'couple time'. As a mum, I love having that occasionally.
I do understand why the word 'childless' might have a stigma to it still, though, as it used to be used to describe 'barren' women.
One thing I think is true, these terms are only really used to define women. It's as if a woman's whole existence is defined by whether or not she's had children, regardless of what else she may have achieved in her life. That just isn't true of men.