I think it's because the average female tends to have a seemingly innate desire to produce at least one child, those who don't are considered abnormal, in denial or bitter over being female.
At least, that's been my experience as a childfree woman.
No, I don't hate kids; I simply don't like long-term exposure to them and have no desire for any of my own. It's like accusing someone of hating cats just because they don't want any.
No, I don't hate being female. I sometimes hate certain parts of my femininity because they give me so much grief, but I have no desire to be male.
No, the desire to remain childfree does not mean I am immune from the desire to share sexual intimacy with my husband. Just because I refuse to welcome pregnancy each time we sleep together doesn't mean I hate my husband, myself, children or those who have them.
I can tell people what I DO hate though, and that's people who refuse to believe any woman would want to limit the number of children they have or outright not have any. I hate people who think the most important function a woman can fulfill is childbirth.
In short, I hate people that try to push their own reproductive and sexual preferences on everybody else because they believe if it's right for them, it must be right for everyone.
If every fertile woman on the planet wanted children or subscribed to the more radical idea that their worth depends on how many offspring they have, there wouldn't be a tree left in sight and everybody would be living like roaches in garbage.
It's just as natural to NOT want children as it is to want them. It's called balance, and I for one am tired of certain kinds of people (I'm sure I don't need to explain which kinds I mean) treating women (and men) who do NOT wish to reproduce like abominations.
I would also like to point out to the birth peddlers that every child I and my fellow childfree people choose not to have leaves another slot open for THEIR children to have better opportunities. You'd think they'd appreciate their children having less competition in the future when they apply for college or work, or when they search for real estate. I don't think these ingrates need to be whining about our choice to decline reproduction.
Quality, not quantity.