Question

Why is sex important in human life?

Answer 1

Biologically, sex is important for genetic diversity, which significantly improves evolutionary adaptation in species. This is true despite the usually very high energy costs of sexual reproduction to the individuals in a given species.

Based on your question details, I don't think the biological role of sex is the answer you are seeking. Sexual intercourse and intimacy carry a variety of meanings for individuals. 

Sex can promote social engagement and unity. This I think is its highest expression. The desire for sex brings people out of their shells and forces them to find strategies to successfully attract and maintain sexual contact with those they find attractive.

Sex can provide pure physical pleasure and relief from tension in the way that having a good meal can be pleasurable and sate feelings of hunger.

Sex can be used to reward others who have given something of value. Oftentimes, the thing of value is just a physical look that is appreciated, but intelligence or empathy can also be rewarded with sex, if the sex is valuable to the person who it is being awarded to.

Sex can boost one's sense of self-worth. If one is physically endowed or skilled, having a wide variety of satisfied sexual partners and admirers can serve to reinforce self-worth and even attract significant resources to the individual that they otherwise would not have access to.