Sex creates a physical bond within a relationship. Sex is essential within a relationship to form a connection.

Plus, it's an enjoyable, exhilirating activity for both parties (if you do it right).

Also exercise.

When an intimate relationship, sex can be a spiritual, mental and emotional expression (hard to explain but I hope you get the drift). It's where two people can really be intimate and enjoying at the same time.

Jacking Off = You + Your Hand
Sex = You + Her Hand j/k You + Her

See the difference? It's one of the ways to maintain a healthy relationship although not really mandatory in some cases. Sex in a "real" loving relationship surpasses physical meaning.

Sex is like some sort of drive, like a certain specification humans have. Although other mammals, protozoa, plants and fungi, etc., have their own means for reproduction, humans make their means of reproduction rather different (in their own perspective).

For example, the internet demonstrates sex as if it where like ogrish.com, and like some sort of temporal enjoyment for a person rather than a couple. (So sad)

But, sex, in a relationship such as bf-gf and as what sex is really ment to be, is the highest point of union. Sex is an excersise of love, in which both beings unite into a single being (as if the humans left behind their bodies and their souls became one), and such creates a great moment for the both of them, in their relationship and etc.

People in this world, including the internet, make it look like a snuff theater, like gore, filthy content for our filthy minds and purposes (so sad). But really, sex is an enjoyable thing. It even shows you how humane you are (if the couple enjoys it).