Quite simple and streight, if majarity of people choose not have any child, human being as whole will finish its life soon - it's like a suicide of one person! Extremism of freedom has caused the declining of Westen culture in the past decades as we all know. BTW, I'm an Asian.
I think some people are better off without kids. Take a look around & see all the unwanted children, it's not fair to those children. I am a teacher & I have enough kids to look after; when I am home, it's my time. I can put more effort into all my students.
The number one drive for every species on the planet is to reproduce, and this drive is handed down to the offspring of any one individual. If an individual doesn't want offspring, they don't reproduce of their own free will, and are therefore eradicated from the gene-pool. Because of this it is unnatural to not want children or offspring, because the want of offspring has been handed down through the generations, while the opposite has been completely eradicated.
It takes many types of people to make the world a successful place, and not everyone was meant to be a parent. Many wonderful teachers never want to have children of their own but they are a wonderful positive influence on many other people's children. How can this be unnatural?
Everything is natural because we are a part of nature.
Every action you take (and your thoughts and feelings are actions too) is happening in nature.
Talking about Is X natural/unnatural? is pointless. Everything is a part of nature, every invention humans have created, even the ugly, evil, terrible things, those are also natural, as is any response to try and stop them. Judging something based on "is it natural?" is absurd.
We were born with a sense of free will and we shouldn't deny other people that right. People have very good reason not to have children in today's society and telling them it is unnatural not to have children does not make your opinion any better than theirs. If they don't want children, simple as that. Why should you care?
Some people know that there will be problems if they have children. They may be afraid of passing on genetic defects or other problems to possible offspring. Some individuals simply prefer the freedom to live free of children. There is nothing abnormal about this. People need to be free to make decisions without being deemed unnatural.
Not wanting children is not unnatural. It is perfectly normal to not want children, especially when you consider modern society and the consequences of having children in modern society. Not having children saves you a lot of time and money, and allows you to have much more freedom. Some people may also not like contributing to overpopulation.