Jesus came to restore creation to the purity of its origins. In the Sermon on the Mount, he interprets God's plan strictly: "You have heard that it was said, 'You shall not commit adultery.' But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart." What God has joined together, let not man put asunder.
The tradition of the Church has understood the sixth commandment as encompassing the whole of human sexuality.
Source: Catechism of the Catholic Church (Vatican.va)