The missionary sex position is a the "usual" position of a male on top of a female during sexual intercourse.
Where did this term "missionary sex position" come from?
When Christian missionaries first arrived in Hawaii they found that the natives of Hawaii would have sex in the open in a wide variety of positions and styles. They would have sex in "doggie style", "ride on a horse style" and "69 position" etc.
Seeing that, the Christian missionaries thought that was a disgusting heathen animalistic action, so they instructed them that Christian sex MUST be man on top woman below and nothing else, hence "Missionary Sex Position".
But in reality, different sex positions can enhance sex life and make sex more exciting. I mean who has sex in one position nowadays????