According to experts on the matter, women have wider hips than men so they can bear children. In other words, pregnancy and childbirth require wide hips to allow for safe gestation and delivery of the baby.
A woman's hip size is influenced by hormonal, genetic, and nutritional factors, not by whether or not she's had sex. They tend to widen during puberty. The pubescent woman's body starts to secrete more estrogen as she goes through the process of sexual development.
This hormone distributes fat in the body and, during adolescence, carries a greater amount of fat to the hips. During this stage, many women also start to be sexually active. That's why wider hips are often associated with sexual activity, giving rise to the myth.
On the other hand, wide hips can also be caused by genetic factors, so it's common to see that women from the same family all have more prominent hips. Furthermore, diet, which can lead to weight gain or obesity, can also cause women to have fuller hips.
Although there doesn't appear to be any correlation between sexual activity and hip size, interestingly, a study published in The Archives of Sexual Behavior concluded that women with wide hips had more sexual partners and casual encounters.
According to the researchers, these results could be explained by the fact that, because having wide hips facilitates childbirth, women who have them see sex as less of a threat to their health.