You have to laugh at life. Or evolution. Or whatever. Because for both men and women, the sources of a deeper, whole-body orgasm are not where you'd think they should be.
In the case of women, the spot that's considered the font of the female orgasm, the clitoris, is completely mislocated. Most women probably lament at least twice a week that the clitoris is not placed just inside the vagina. The same goes for the G-spot (or urethral sponge), which is in a tricky spot inside the vagina just beyond the curve of the pubic bone - and the penis typically misses it altogether.
So it is that the prostate gland (aka his P-spot), a highly sensitive organ akin to the woman's G-spot, is reached through the anus. While the function of the prostate gland is to produce ejaculatory fluid, it is also a source of sexual pleasure for many guys because it is located on the nerve pathways between the brain and penis. Like the G-spot, it is linked to the pelvic nerve (the clitoris is linked to the pudendal nerve), which is why the orgasm feels deeper than the usual penile orgasm.
Why stimulate the prostate?
Many men can orgasm solely from massaging their P-spot. And when the prostate is stimulated at the same time as the penis, the sensation is doubly pleasurable because the prostate nestles at the base of the penis. Playing with both sites means that the two ends of the penis are receiving stimulation, which is more intense. What's more, the anus itself is an erogenous zone because the anal cavity is rich in sensitive nerve endings, so stimulating the cavity can be exquisitely pleasurable.
Yet, just as some women aren't moved if their G-spot is massaged or find the sensation positively irritating, not all men enjoy having their prostate stimulated. We are all unique. However, what many couples discover after years of the same bedroom routine is that playing with a wider range of stimuli can broaden the sexual experience. People who are willing to be adventurous and experiment are often surprised at how enjoyable anal play can be - for both partners.
Isn't the rectum 'dirty'?
Faeces passes through the rectum but isn't stored there, and the cavity self-cleans. Unless he's had a recent bowel movement, it's likely that only traces of faeces may be encountered. If he washes beforehand and inserts a soapy finger (preferably using a soap-free cleanser) to clean gently inside, most traces will be removed. With anal play, it's always handy to keep some unscented wet wipes or baby wipes handy nearby.
The person doing the stimulating should use a barrier, such as a surgical glove, finger cot (used to clean babies' teeth) or a condom. Pop one on the finger or prostate vibrator and use lots of lubricant (preferably a thicker anal lube) as unlike the vagina, the rectum does not produce any natural lubricant to reduce tearing or 'burn' from friction. Because sexually transmitted diseases including HIV/Aids and other infections such as Hepatitis B can be transmitted in unprotected anal sex, it is advisable always to use a latex barrier in anal play.
How to stimulate the prostate
The first approach needs to be slow and gentle, using plenty of water-based lubrication. The more aroused a person is the more pleasurable it will feel, so foreplay is essential. Or the P-spot can be stimulated during oral or manual sex to double his pleasure. Start out with a small object like a pinky finger, narrow vibrator or string of anal beads. (Note that the rectum curves forward for the first four or so inches and then curves towards the back, so only flexible objects that can follow the curve should be inserted beyond four inches.)
The prostate is stimulated through the front wall of the rectum (the belly button side). The spot is located about three inches in and it typically feels like a firm bulge about the size of a walnut. It usually responds to firm side-to-side stroking or a circular massage, while men often find vibration from a curved sex toy highly pleasurable. Moving the finger in and out can also be enjoyable.
As with any sexual activity, the pleasure is intensified if more than one area of the body is stimulated simultaneously. Playful experimenting while requesting feedback usually yields great rewards. And if his P-spot is correctly stimulated, he stands to experience an orgasm that is divinely earth-shattering.
[Sources: 302 Advanced techniques for driving a man wild in bed, Superhotsex, The Good Vibrations Guide to Sex and “The last lovemaking taboo lifted?” (MSNBC)]