Each belongs to the other. The wife's body does not belong to her alone but also to her husband. In the same way, the husband's body does not belong to him alone but also to his wife. Sexual intercourse is an intimate expression of affection between a husband and wife. Either can initiate intimacy.
Intimacy is more than an act of biological mating.
Marriage is the one and the only place that God has provided for sexual union to take place. In that setting it becomes a powerful symbol of the love between Christ and the church, a pure sharing of joy and delight in one another that is a gift from the hand of God. Outside those boundaries, it eventually becomes destructive. The pattern is Jesus who gave Himself to only one bride, the church. Men are to be sensitive to their wives.