Abuse of the system

My anatomy and physiology textbook includes a thorough, well written chapter explaining the perfection with which female and male bodies are designed to have children together. It then lists most of the ways you can interfere with the process. It does mention the possibility of a natural, uncontracepted married relationship, but contrary to the impression you might get from the first part of the chapter, it’s not an excellent way to have a baby, 85% successful within the first year. It’s just a very ineffective means of contraception, with an 85% rate of failure.