What was then defined as the penetration of the penis to the vagina has now had varieties like oral and anal sex. Another common form of arthritis RA is an autoimmune disease. It happens when your body's immune system attacks body tissue. These attacks affect the synovial tissue in the joints, a fluid that nourishes the cartilage and lubricates the joints. Sexual intercourse is also a good form of exercise since the act requires body mechanics and it is also known to stimulate chemical reactions inside the human body that is highly beneficial