text: I would assume we are speaking about the scientific purpose for sexual intercourse.  Of course, humans, having advanced brains, have come up with many purposes for sex. \n\nScientfically, humans procreate through sexual intercourse otherwise known as internal fertilization.  Some animals reproduce using external fertilization, such as frogs.  Other organisms such as bacteria reproduce by copying themselves in a process called budding.  This is much like the cell division in our own bodies.  \n\nThe more complex an organism is the more advanced there reproduction is.  Some animals have many offspring to ensure that their DNA propagates through a population.  In other words, there is safety in numbers.  The less complex an organism's reproduction is the less complex the animal is.  Therfore the animal's offspring has a less chance of survival and reproducing themselves.  This is why some animals have so much offspring and why some only have one offspring.\n\nA parent is not environmentally 'fit' if it's offspring do not reproduce.  The animals that survive and have offspring that reproduce, have the DNA that is 'fit' for their environments.  Why do you think parents are so happy when they become "grandma" and "grandpa"?  They have been successful in their reproduction, or for the purpose of this post,  with their sexual intercourse.  Our survival and 'fitness' is not only dependent on DNA but on our ability to survive in our complex social world, which requires a strong supportive environment.  Scholars cannot agree  about how much DNA and the environment play a role in human survival.  I can assure you though that both are very important.\n\nAs for the evoultionary aspect, any organism has to reproduce in order to propagate in the world.  In order for organsims to reproduce there has to be an inate desire to do so.  This desire is controlled by a number of chemicals in our bodies, many of which are hormones.  Phenotypes (appearance) have also evolved to make humans more sexually attractive to each other, which helps ensure that sexual intercourse will take place.  For example, our loss of hair on our faces and bodies is much more aesthetically pleasing and soft to the touch.  This loss of hair also allows us to see many more facial expressions that convey non-verbal messages more clearly to a would be partner. \n\nThe simplest form of life, which we are said to have originated from, is bacteria-like in nature.  As organsims evolved so did their reproduction.  Internal Fertilization is much more successful than external for the simple reason of it taking place 'indoors'.  \n\nEvolution is change, plain and simple.  This change for humans takes thousands of years, but can be noticed in days or even hours in bacteria.  This is the doctrine of uniformitarianism which is used in geology, but can be applied to other fields of science.  This doctrine states that present processes help to explain the past.  If bacteria can change so quickly and adapt and even resist some things that killed their ancestors, this is clearly a sign of change and hence is called 'evolution'.  What you might ask is because bacteria make an exact copy of themselves so how could they ever change?  Usually bacteria make exact copies, however there are millions and millions of bacteria and every so often there is a random genetic mutation in some of them.  It just so happens that they get lucky because that mutation allows them to survive and pass that trait on to their offspring.    \n\nAll organisms are different.  Humans sexually desire each other and have intercourse for many reasons.  But, scientifically there is one reason and one reason only, Internal fertilization (sexual intercourse) is the most successful and advanced means of reproduction and is a result of trial and error (evolution).  It helps to ensure our propagation through the population.