A trait may not be morally good just because it assures survival. Further, if people's lives contain intrinsic good then it is good that people live and existence is good. Many people do not want children and the duty to reproduce would impose much unhappiness. If people have children out of duty they may not be good parents and their children would be less happy. If both steps two and five in the natural law theory are not necessarily logical then it very well may be that people are doing nothing wrong by not having children and contraception is then not morally wrong.
Sexuality is a private matter.
Public policies arise as a result of such questions. Since the invention of agriculture no important culture has left decisions about reproduction entirely up to individuals. In the United States both legislative and court decisions have addressed such questions. One major policy that affects sexual behavior is the 'right to privacy' established by the 14th Amendment to the Constitution. A state can restrict that privacy only by a clear showing of harm or by demonstration of a compelling state interest.