Scientific interest in human sexuality came very late in the 20th century. For many years, the prevailing view was that it was inappropriate to deal with these issues and that the sexual act should proceed "naturally" without the intervention of a doctor or even without prior education. But what does "normally" mean? How do we know that we are not deviating from this norm? This text presents a description of two main concepts about the sequence of changes that occur in the human body during sexual activity.