If we take a "gene's eye view" of evolution, the basic principles of "survive and reproduce" should drive human behavior in a predictable direction: we should produce as many children as possible. The instinct to reproduce isn't contained solely within the convention of heterosexual marriages and relationships. The innate drive to reproduce motivates pre-marital, extramarital, and homosexual behavior too. Some of us may be childless by choice but very few of us are sexless by choice.
Despite the biological instinct to procreate ceaselessly and despite strong encouragement from religious authorities to bring more Christians and Muslims into the world, most people, even the most religious among us, don't reproduce to our maximum potential.
Our bodies may tell us to reproduce without constraint but our reasoning ability warns us against that idea. Modern couples typically agree to produce a manageable number of children. Context influences the thinking. In agrarian societies the production of children adds to the family labor force; reproduction is a biological and cultural necessity. But for modern, non-farming families, limiting the number of children is a response to other kinds of economic, cultural, and personal conditions and preferences.
The decision not to have children represents the power of human cognition and the freedom that is granted individuals in modern Western societies. Human behavior is influenced but not determined by our instincts. Culture and personal initiative mitigate biologically-driven behavior in many aspects of life, not just sex and reproduction.
Those who are "childless by choice" know that decision can bring great benefits. The benefits may not be strictly evolutionary, but many of us who have chosen not to bring children into the world believe that bold decision has sustained and improved our quality of life. On a grander scale, however, there is real evolutionary advantage to curbing excessive population growth. The best chance for survival of any species is to make sacrifices for the good of the group--like limiting population growth.
Part of what separates us from all other species is our ability to consciously disobey our instincts. We are the only species that can make that decision with conscious awareness.
Evolution has no purpose. It is not a search for the perfect or even the better. Evolution is simply the accumulation of adaptations made by organisms to the environments they inhabit. The decision not to have children is one of those cultural adaptations carried out at the level of the individual person or couple.
Whenever I personally thought of having children I rejected the idea because at every stage of my life I thought things were just fine the way they were. I feel absolutely confident that all those decisions along the line were right for me.
For great information and provocative discussions about living childfree, check out
Laura Carroll's excellent blog www.LauraCarroll.com