Nature Said Anus is For Life Don't Fuck it Fools.

Susan James:   So the body had evolved strong signals to motivate us to defecate and to control the process. The rectum and anus are rich in nerve endings which both send stimuli to the brain and control the muscles of the rectum and sphincter. When the rectum gets full of feces, the nerves send strong signals to the brain, giving us a sense of fullness and stimulating the walls of the muscles of the rectum to contract. At the same time, in order to prevent the expulsion of feces in an uncontrolled manner, the muscles of the sphincter tighten. When we are ready to defecate, the muscles of the rectum contract strongly and the muscles of the sphincter are forced open to allow the feces to pass. Throughout this process, the nerves in the rectum and sphincter are sending strong signals to the brain, helping us monitor the process.

If we are honest, we must admit that the process of defecation is usually pleasurable, even if the by-products are not. This all makes sense from an evolutionary perspective; we need to poop, so it makes sense that it feels good to poop.

Anal sex reverse engineers this entire process to produce pleasure. The penis stretching the sphincter open and giving sensations of fullness in the rectum stimulates all the nerves that makes pooping feel good. Unlike pooping, however, which lasts for a short time, anal sex continues on for some length of time, flooding the brain with pleasurable sensations. The proximity of the anus and rectum to the crura (legs) of the ciltoris and G-spot in women and the prostate gland in men also means that pressure and thrusting in the rectum produce additional pleasure.