Men and women have very similar anal structures. The anatomical aspects of the butt which are not completely paralleled in the opposite sex have counterparts that are located and stimulated comparably. Because our asses are relatively identical, we all can enjoy the same kind of stimulation and reach similar levels of pleasure during anal play.
General Anal Structure:
The physical structure predominantly stimulated during anal sex involves the anal canal and the rectum. The anal canal is roughly two inches long, depending on the person, and is made up of the external and the internal sphincters. The external sphincter (anus or asshole) is the outermost muscular ring and has an outside appearance covered by folded skin; giving its puckered look. Not far beyond the anus lies the internal sphincter leading into the rectum.
We can consciously relax/tense the external sphincter, but the internal is controlled by the autonomic nervous system, which means its movements are involuntary. You may be wondering, "Even if I can relax my asshole to penetrate it, how can I comfortably get past the involuntary inner sphincter?" The two sphincters seem to work in tandem; if you can ease the external sphincter, the internal usually follows suit.
The anal canal acts as a gateway into, or out of, the rectum. The rectum is a tube about 9-10 inches long that leads from the colon (large intestine) to the anus. The rectum is not a straight tube; in a cross section of the pelvis, the rectum‘s curvature with the coccyx (tailbone) looks like a caterpillar relaxing on a lounger. The rectum has a lazy S-shaped structure, within that structure there are several valves which fold horizontally like a cursive Z or a winding road. The first structural curve is located not far behind the anal canal; it is caused by the pubo-rectal sling, which supports the rectum by anchoring it around the pubic bone. The second structural curve is at the top of the rectum where it meets the colon. The structural curves of the rectum are more exaggerated and anatomically consistent than the subtle curvature of the slaloming valves, which vary in size, angle, and quantity from person to person. It is important to become familiar with your rectal curves in order to achieve sexual pleasure from anal play.
The muscular structure of the pelvis is vital to all sexual stimulation, especially anal sex. Your lower pelvis and the area known as the pelvic floor are composed of muscles that work together for sexual stimulation, intercourse, urination, defecation, and childbirth. The area is a diamond shape; the top point is the pubic bone just above the clitoris/penis, the bottom point is the tail bone just behind your anus, and the side points are where the legs meet the pelvis parallel to the space between the vaginal opening/scrotum and the anus (this area is known as the perineum).
Many people enjoy the sensation of the sphincters opening and closing, this is because there are so many nerve endings packed together at the anus. The sensation travels through the nerves and the pressure can be felt throughout the pelvic muscle groups. There is a band of muscular tissue making a figure 8 inside the pelvic diamond, one loop around the anal sphincter and the other around the genitals.
The upper muscular loop incases nerves that lead directly to the clitoris and vaginal opening in women; in men the nerves are connected to the head of the penis, the tissue beneath the base of the shaft (bulb or root), and the connecting tissue of the scrotum. Because the genital and anal loops are united, when the anus is touched or penetrated the sensation is felt throughout the genitals. The area in the middle of the figure 8, the perineum, can also be stimulated to send sensations through both genital and anal loops.
The perineum is a major part of Tantric and Taoist sexual meditation/practice. It can indirectly stimulate the prostate in men, the G-Spot in women, and the root of the genitals in both. The male root is the tissue below the base of the penis and in women it is the tissue beneath the outer labia and vaginal opening. By focusing attention on this area, working with the band of muscles that compile the figure-8 and the surrounding tissue, you can increase sexual stamina, genital strength, and help prevent urinal and fecal incontinence. These are referred to as the pubococcygeus (PC or pelvic floor) muscles; they contract rhythmically during orgasm and help in the expulsion of ejaculate. The stronger your PC muscles are, the more intense your orgasms can be. The work out plan for the PC muscles is known as Kegel exercises.
P-Spot and G-Spot:
Both the P-Spot and the G-Spot refer to the locations where we can actually stimulate the tissue around the glands which produce ejaculatory fluid in men and women. Many people use these terms in reference to the prostate and paraurethral glands themselves; this is technically incorrect but widely accepted.
The prostate is the most ignored source of sexual pleasure in men and what a shame this is because the sensations created by the P-Spot are potentially mind blowing and very different from penile pleasure. This walnut-sized gland is found in men only; in women it is referred to as the Skene's or paraurethral gland located in the urethral sponge. The prostate rests just in front of the rectum and just below the bladder; wrapped around the urethra. Its function is to produce fluid to help ejaculation, the prostate swells with the fluid during arousal. This white-ish clear fluid is made to lubricate the movement of sperm and protect it from any acidic urine left in the urethra.
The prostate is often ignored due to fear of anal penetration, but it can be stimulated indirectly by massaging the perineum. Pressing firmly against this spot during other forms of stimulation is a Tantric sexual technique that has caused many a man to twitch with pleasure. Perhaps you want direct prostatic stimulation; you can manipulate this sensitive spot by making a "come here" motion with the index finger against the front wall of the rectum (2-3 inches past the sphincters). The prostate will have a spongy texture when a man is aroused, which will make it distinctly different within the rectal wall. The prostate has been called the "Male G-Spot" because its point of stimulation in the rectum is analogous to the G-Spot's location in the vagina and the G-Spot serves a similar physiological purpose.
G-Spot is short for Grafenberg spot, but the more anatomically correct name for this piece of female genitalia is paraurethral sponge. This erectile tissue is located just behind the pubic bone and, like the prostate, just below the bladder, surrounding the urethra. One of its functions is creating a buffer zone of spongy tissue to protect the urethra during vaginal penetration. The G-Spot can be stimulated directly by pressing firmly on the roof of the vagina, about 2-3 inches behind the vaginal opening. The G-Spot surface inside the vagina ranges from the size of a dime to the size of a half dollar, depending on the woman. During arousal the area swells with fluid and has a distinctly different texture from the rest of the vaginal tissue.
The G-Spot can be indirectly stimulated by anal penetration. The vagina is made of thin, flexible tissue and it lies directly in front of the rectum. The pressure provided by anal penetration travels through the vaginal walls without much resistance creating G-Spot stimulation. The G-Spot has a reputation of being the secret key to unlock the mystery of the female ejaculation.* It is the location of the paraurethral glands (Skene's glands) which secrete fluid into the urethra just above the labia minora. A clear or white-ish fluid can be propelled out of the urethra; this is female ejaculate. This occurs through direct G-Spot stimulation, intense clitoral stimulation, or some combination of both.
*There is much scientific debate whether or not every woman is capable of ejaculation or even has a G-Spot. But there are medical studies suggesting the pores of the paraurethral sponge have a wide range in size; meaning that some women cannot excrete a noticeable amount of this fluid through smaller pores. Another issue is a lot of women resist the physical urge to ejaculate because they fear urination during sex; if those women allowed the urethra to open, they would find the ejaculatory fluid is distinctly different from urine.