Your words disturb me. 'Jesus', 'fucking' and 'ass' are not words I like to see used in the same sentence.
its to remove you from the world, its problems, your concerns into a truely divine feeling of saftey and accomplishment by streaming endorphins shooting through your body everywhere at once. as result you are healthy, your loved one will feel accomplished (unless solo) and the greatest gift of all can be brought into the world. it fucking feels good mang.
The orgasm evolved as a way of encouraging people to have more sex. People that enjoyed sex more had more sex and, therefore, more offspring that enjoyed sex more. Your ancestors fucked long and hard so you could get off someday, don't squander the gift, ha.