Why do we have long sex?

As an evolutionary researcher, all the talk about how long sex lasts makes me wonder: Why does sexual intercourse take time too? All intercourse aims to insert sperm into the vagina. Then what is the excitement in bed? Instead of inserting and removing the penis hundreds of times during intercourse, why not include it once and after that we can spend our time drinking lemon?
Before you answer, that it's fun, keep in mind that the evolutionary process does not care about fun things solely; the evolutionary process of designing everything becomes fun just because it helps our ancestors pass on their genes to future generations.
For example, even though we enjoy eating food, we will not chew food for five minutes just because we like it. It will make eating activities inefficient, and we evolve to understand that chewing food for too long is disgusting.
Why we make love long is a complicated question and there is no clear answer, but the clue may lie in the shape of the penis itself. In 2003, researchers discovered-by using vaginal, penile, and artificial sperm - that the tip of the penis head actually takes sperm that previously existed in the vagina.
This suggests that repetitive penis movement in and out actually aims to eliminate other male sperm before ejaculating, to ensure that their sperm first arrive at the egg. This then explains why men feel pain when they push their penis back after ejaculating, because it means that the movement is at risk to eliminate their own sperm.

So what needs to do with this information? My advice should not be too thoughtful when having sex.