Source: Journal of Sexual Medicine: “Pain experienced during vaginal and anal intercourse”, Debbie Herbenick, 2015.
Due to impact on the cervix and surrounding muscle tissue, conditions like endometriosis, bladder complications, postnatal prolapse, or even "mechanical" (anatomical) fit, etc can contribute to discomfort during and/or after penetrative sex. Even stress from a previous trauma can also activate physical distress.
It takes an average of 6-7 years to get a diagnosis (if any), and solution options are often invasive, prohibitively expensive, or non-existent.