For men in patriarchal cultures, it is, but patriarchy has only been around for about 5k years. In addition, there are all sorts of cultures around the world today where sexual fidelity is neither expected nor prioritized.
Also, sexual exclusivity isn't a very smart evolutionary strategy. Multi-mating gives women in particular, access to a greater chance of having healthy offspring.
Aside from the fact that many primate females are far from sexually reticent or choosy, across all species, mating with several males confers a significant evolutionary benefit.
Gowaty describes the benefits of multiple mates as an answer to the never-ending evolutionary struggle against what may be the world’s greatest predator: disease.In this illness-driven arms race, organisms that produce offspring from multiple mates are more likely to produce some children with the right antibodies to survive the next generation of viruses, bacteria and parasites. Source
Polyandry was normal in pre-contact Polynesia, particularly for high caste women and still takes place in the Indian Himalayas and in parts of Tibet. In Lowland South America, and in Africa partible paternity, where two or more men mate with a woman for the purposes of producing a child, is common in many cultures. Spreading fatherly feelings throughout the group helps to maintain solidarity and cohesion as well as promotes the wellbeing of a greater number of children. Reproductive fitness (the chance that offspring will, in turn, produce their own offspring) is enhanced by cooperative alloparenting of this kind where several adults take an active interest in the lives of children.
When seventeenth-century Jesuit missionary Paul Le Jeune lectured a Montagnais Indian man about the dangers of the rampant infidelity he’d witnessed, Le Jeune received a lesson on proper parenthood in response. The missionary recalled, “I told him that it was not honorable for a woman to love any one else except her husband, and that this evil being among them, he himself was not sure that his son, who was there present, was his son. He replied, ‘Thou hast no sense. You French people love only your own children; but we all love all the children of our tribe.’”
Ryan, Christopher. Sex at Dawn (pp. 122–123). Harper Perennial. Kindle Edition.
The Warao of Brazil periodically suspend marriages and have ritual relations called mamuse. During this time, adults are free to have sex with whom they please. These relationships are considered to be honorable and thought to have a positive effect upon any children that might result.
Young women cheat all the time. Thinking that you can somehow control a woman better if she's young is just one more anti-scientific patriarchal narrative that needs to go.
