When Penetration is Painful or Impossible
Hitting a Wall
Depending on the intensity, penetration issues will range from minor burning sensations with tightness to total closure of the vaginal opening with impossible penetration. Many couples describe the feeling of "hitting a wall" when attempting intercourse.
This is often referred to as vaginismus, a pelvic pain disorder. Symptoms range from burning with tightness during sex to impossible penetration, even spasms in other body muscle groups.
Examples of the Effects of Vaginismus
Partial Penetration
(Image 1)
Penetration may be possible, but some women experience periods of involuntary tightness causing burning, discomfort, or pain.
Impossible Penetration
(Image 2)
As the man approaches the woman to attempt intercourse, her vaginal muscles involuntarily tighten the vaginal entrance, making intercourse virtually impossible.
What is Vaginismus  
Vaginismus is an involuntary contraction of the muscles surrounding the entrance to the vagina, making penetration painful and even impossible.
This involuntary vaginal reflex is a limbic system reaction, triggered by attempts at intercourse with the entry or approach of a penis to the vagina and, or by the insertion of other objects such as a tampon, a doctor's finger, or even your fingers. The spasms constrict the vaginal opening, making it virtually impossible to have intercourse.
Common Symptoms
Burning or stinging with tightness during sex
Difficult or impossible penetration, entry pain, uncomfortable insertion of a penis
Unconsummated marriage, not by choice
Ongoing sexual discomfort or pain following childbirth, yeast/urinary infections, STDs, IC, hysterectomy, cancer and surgeries, rape, menopause, or other issues
Ongoing sexual pain of no unknown origin, with no apparent cause
Difficulty inserting tampons or undergoing a pelvic/gynecological exam
Spasms in other body muscle groups (legs, lower back, etc.)
A Limbic System Reaction 
Muscle Reaction
Naturally, sexual messages will vary among women, but many women will experience a conditioned reflex response in the vaginal muscle group during penetration. Even if there's a desire for physical intimacy or a will for tampon insertion, the limbic system tells the body "I'm going to protect you from harm" triggering pelvic floor muscles to go into spasm. It's an associated reaction. 
There's a part of the brain that listens for things that may be a concern for the body. When the body anticipates pain, it sends a signal to brace and protect, causing muscles to tighten.
Sexual pain does not discriminate. Uncomfortable or impossible penetration can affect women in all stages of life — even women who have had years of comfortable sex.