Why do doctors tell you to wait 6-8 weeks to have sex after you have a baby?

Look at the process. Your uterus is about the size of a pear, and it stretched out to the size of a basketball. At delivery, the outlet, which is the size of a coke bottle is stretched and torn as a kid with a head bigger than most grapefruit passes through it. You might feel fine, but it takes the body a while to repair all the stretching and tearing. ^ weeks will return most tissues to a stable sate.
Post partum sex. For several reasons: if the patient had sutures placed such as an episiotomy, laceration repair or c-section then it takes about 6 weeks for the sutures to be absorbed. 6 weeks is also about the time it takes for your body to go almost back to its non-pregnant physiologic state.