###Human: Full penis penetration causes pain in vagina. Why? ###Assistant: Hi. Painful penetration in women may be caused by any injury or lacerations in vaginal walls, infection of the genital tract, forceful or violent sex, lack of lubrication in the vagina. I think you should do a lot of foreplay before penetration so that her vagina may get lubricated naturally. Also, it is highly recommended to use a water-based lubricant like KY jelly. Always be gentle to your partner while having sex. That is the key.