The starting point for a Christian understanding of human sexuality is the belief that all humans are made in the image of God:
God created man in His own image, in the image of God created He him; male and female created He them.Genesis 1:27
The creation story in Genesis shows how male and females are intimately connected to each other:
So the Lord God caused a deep sleep to fall upon the man, and while he slept took one of his ribs and closed up its place with flesh; and the rib which the Lord God had taken from the man He made into a woman and brought her to the man. Then the man said, 'This at last is bone of my bones and flesh of my flesh; she shall be called Woman, because she was taken out of Man.' Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh.Genesis 2:21-24
This story suggests that man and woman unite to become complete.
Saint Paul reminds Christians that:
Do you not know that your body is a temple of the Holy Spirit within you, which you have from God? You are not your own; you were bought with a price. So glorify God in your body.1 Corinthians 6:19-20
This teaching is important because it refers to the responsibility Christians have for what they do with their body and how they treat it. The teaching has a strong implication for a Christian's sexuality.
You shall not commit adultery.Exodus 20:14