The wearing of delicate tissues of an insufficiently lubricated vagina through penetration or rubbing can cause abrasion, small cuts, infection and increase the risk of HIV. Other possible causes of cuts results from stretching the vagina walls without relaxation (relaxing the vaginal muscles in proper posture promotes stretching). In preparation for medical examination or sexual play, use vaginal lubrication, relaxation techniques, or other professional adviced techniques to reduce abrasion and tear. Pausing a moment for your special someone to get into the mood will also increase lubrication.
Small tears, sores, and cuts can prevent pleasurable sexual activities, but will usually heal naturally within a few days. Consult a doctor if it doesn't heal.