Men:women are born at 107:100. There are about 7% more men than women in the world at birth. This only evens out by age 40. This is due to suicide rates among men being wildly higher.
Most men and women are monogamous - but many people date. A few men sleep with many women. Many men never see a date or any sex. 25% of men will be childless. 15% of women will be childless. It should get kinda obvious how this works even with a monogamous society. Women can get sex when they want and not commit to someone even in a monogamous society. The attractive men can choose to be non-committal and sleep with many women. Eventually - many women will commit to a man but it will be to a man who had very few partners whereas the woman might've slept with many of the men who get to sleep with many women.
Basically - the idea is that a small subset of men are sleeping with all the women. The women might settle down eventually but they will inevitably settle down with a man who did not have many sexual partners.