The standard theory is that the human lineage became the dominant species on Earth because we descended from the trees and adopted a permanent upright posture, leaving our hands free for other purposes, rather than for hanging from tree branches.
Allegedly, a pattern developed that gave humans a reproductive edge by freeing mothers of young children from having to search for food, as this would be handled by males. This sharing of responsibilities allowed females to become pregnant at shorter intervals, and at the same time take care of a considerable number of offspring.
For males to act as caring husbands and fathers, the adoption of an upright posture was essential to efficiently manage the transportation of food for their wives and children, left behind in a safe location.
An incentive for males to become food suppliers was the availability of females for sex at any time, not just during fertile periods.
Evolution and natural selection favored hypersexual males, and females that could raise several successive cohorts of offspring. By contrast, the usual mammalian pattern is that the next set of offspring is born only after the previous one has been weaned. Humans have adapted to this pattern over millions of years.
Humans have made great progress lately. The emphasis is on “lately.” The modern world started only some 200 years ago, with industrialization and science.
What separates us from the Stone Age tribes of 100,000 years ago is cultural, not biological. We are changing our societies at great speed. But our genetically encoded traits are the same as they were tens of thousands of years ago.
The genetically encoded pattern of behavior is that females raise the children, while males find food and provide protection.
In human evolution, the most successful reproductive strategy for females has been to secure a male who would provide food and protection for her and her offspring. For her and her offspring, not the offspring of other females. Female jealousy is a biological trait.
Essential male traits are also genetically encoded. The most successful reproductive strategy for a male is to spread his genes with as many women as possible, provided many descendants survive. Males want to build harems, formal or informal.
The equation “sex for food” is inherently understood. It applies to wives and prostitutes.
Sexual relationships are important for both women and men. In societies focused on basic survival, there are clear and simple patterns for orientation. Females have sexual needs, but they select males in accordance with their qualifications as providers and protectors. Males aim to improve their qualifications by seeking wealth and success.
In an affluent society, people lack such clear orientation, and everything becomes confused.