
I remember that A man called Cain was the fist sinner 'couse he killed his brother. But this story's about sex?

Cain was the first-born son of Adam and Eve, making him the first human child ever to be born. Like his father Adam, he became a farmer and worked the soil.
The Bible doesn't tell us a lot about Cain, yet we discover in a few short verses that Cain had a serious anger management problem. He bears the unfortunate title of first person to commit murder.
The story of Cain and Abel begins with the two brothers bringing an offering to the Lord.
The Bible says that God was pleased with Abel's sacrifice, but not with Cain's. As a result Cain grew angry, dejected, and jealous. Soon his fierce anger led him to attack and kill his brother.
The account leaves us wondering why God looked with favour on Abel's offering, but rejected Cain's. This mystery confuses many believers. However, verse 6 and 7 of Genesis 4 contain the clue to solve the mystery.
After seeing Cain's anger over the rejection of his sacrifice, God spoke to Cain:
Then the LORD said to Cain, "Why are you angry? Why is your face downcast? If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it. (NIV)
Cain should not have been angry. Apparently both he and Abel knew what God expected as the "right" offering. God must have already explained it to them. Both Cain and God knew that he had given an unacceptable offering.
Perhaps even more important, God knew that Cain had given with a wrong attitude in his heart. Even still, God offered Cain a chance to make things right and warned him that the sin of anger would destroy him if he did not master it.
Cain was faced with a choice. He could turn from his anger, change his attitude, and make things right with God, or he could intentionally give himself over to sin.
Finally the anime is over, I hope yo have more and more chap in the near future! I love this manga(⌒▽⌒)