There are good biological reasons for this, since it determines the reproduction of the species and the spreading of individual genes. This is not to diminish its meaning but rather to increase it. Life on earth is a vast tree with myriad branches, on which each of us is a leaf. Sex is the activity through which we join to the growing branches. It links us to all life in the past, and all human life in the future.
Sex is also a mystery, similar to the mystery of existence.
Sex can also be a religious sacrament. Some groups like the Tantrics have extended this principle, to see sex as a link with the totality, a way of abandoning the self in the arms of the universe. Others have gone further still. Pursuing the logic of pantheism to extreme conclusions, they have assumed that, since everything is God, they too, individually, are gods, and can do as they please. So the Free Spirit heretics of medieval Europe were said to encourage every kind of crime, including theft, incest, and murder.