Men tend to fantasize about the size of their penis. They long to till the vagina to a stretching point, to reach deep inside their beloved and batter against the cervix. A few psychologists have lurid reasons for this. Yet it seems a reasonable enough and harmless myth, because the cervix can escape penile battering. At sexual arousal, the inner two-thirds of the vagina increase in length a further two inches. They widen to stretch to a full two inches in diameter. The uterus with its cervix is pulled right up and out of harm's way. This is known as "the ballooning effect."
At the same time, the outer one-third of the vagina swells up. This is due to Vaso-congestion. Sexual arousal causes extra blood to enter and fill up the tissues. The result is a narrowing and tightening inside the entrance to the vagina. There is a clamping of tissue around the penis as it thrusts. This is known as "the gripping effect."
If the average erect penis is 6¼ inches long and 1¼ to 1¾ inches wide, the average aroused vagina will sheath it to mutual satisfaction. The myth of a tiny, tight vagina during the act of love is just that, a myth. The ballooning effect helps the vagina stretch to accommodate the penis in comfort. Only an "unaroused" vagina can offer resistance to deep penetration, (or one which is sore from repeated love making .or infection). If the vagina does feel tiny and tight, then it is not ready for penetration. The woman lacks sufficient desire at that time.
The cervix projects into the upper part of the vagina, with a dip like a tiny trough running around it. This dip has front, back, and side parts called fornices. The back fornix is another mechanism to protect the cervix from battering. It takes the brunt of penile thrusting, and so prevents injury or jarring.
The walls of each fornix are thin, and the uterus, ovaries, and oviducts can be felt through them on a pelvic exam. They assist in fertility if, after making love, the woman remains lying on her back. The semen can then pool in the posterior fornix, and bathe the cervix. This allows more time for sperm to enter the os, the tiny opening of the cervix which leads to the uterus.