You seem to be making an error, common to most people who read the Bible. And that this is error is that they do not comprehend exactly what is going on at the time the Pentateuch was written. The nation of Israel had been in slavery in Egypt for over 400 years. During that time they had gotten away from serving their God, and had been under the influence of a nation which served multiple god's.
Some of the Roman gods were immoral gods, and Egypt at the time was a very immoral country. After God delivered the nation Israel from slavery in Egypt, he had to retrain them and teach them his laws and his ways anew.
The book of Genesis was given to remind Israel of their history up to the time of Joseph which they no longer remembered. Beginning in the book of Exodus, For the most part throughout the second part of Exodus through the book of Deuteronomy, God is giving them his laws and rules for living. Although there is some overlap between the laws and the rules for living for the most part the laws are given as commands. Rules for living have to do with normal day to day life.
Beginning in Exodus chapter twenty verse 1 God gave the nation of Israel his basic laws.
(note) All Scriptures are quoted from the King James translation.
Exodus 20:1 through 17
1 And God spake all these words, saying,
2 I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage.
3 Thou shalt have no other gods before me.
4 Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth:
5 Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;
6 And shewing mercy unto thousands of them that love me, and keep my commandments.
7 Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain.
8 Remember the sabbath day, to keep it holy.
9 Six days shalt thou labour, and do all thy work:
10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
12 Honour thy father and thy mother: that thy days may be long upon the land which the LORD thy God giveth thee.
13 Thou shalt not kill.
14 Thou shalt not commit adultery.
15 Thou shalt not steal.
16 Thou shalt not bear false witness against thy neighbour.
17 Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour's.
The same 10 Commandments are given again in Deuteronomy chapter 5, but some wording is changed in order to make it more understandable
Although these Commandments seems simple enough to us. It was difficult for the nation of Israel having been under the influence of the Egyptians for so many years to truly understand what that meant. For years under the Egyptian rule sexual promiscuity was the rule rather than the exception. What we find in the rest of the Pentateuch is God having to elaborate on the exact meaning of the 10 Commandments in order for the nation of Israel to understand.
In Leviticus, chapter 18 God is giving more explanation to his command that thou shalt not covet thy neighbor's wife, nor anything that is thy neighbor's. He is also telling them that homosexuality and incest are against God's laws. In chapter 18 verses one through 30 God is defining unlawful marriage.(We must remember that in God eyes sexual intercourse constitutes Marriage.) In most religions that is known as consummating the marriage. We need only to look at the story of Joseph Leah, Rachel and their two maids beginning in Genesis chapter 29 to understand the permanence of that union in God's eyes. The children of all four comprises the nation of Israel.
In chapter 19 of Leviticus, we go to a completely different story, in Leviticus chapters 1 through 11 God is giving Israel a new set of commands about how he is to be served. He gives the rules for sacrifice. What animals are to be considered, clean and which are unclean, and he gives all of the instructions for the priest. What God is doing in chapter 19 is that after he has given them all of the laws and instructions on how to serve him; he is giving them a warning about not breaking these laws and instructions.
The laws which God gave always have and always will be in effect:
Matthew 5:17 and 18
Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil. For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
The rules for worship that God gave to Israel. After bringing them out of Egypt do not apply to us, only in as much as we no longer have to sacrifice animals and we now have only one high priest which is Jesus Christ.
We are still bound by the law, even though Jesus fulfilled the law. Fulfilling the law is not the same as doing away with the law, every time that we break one of God's laws. It is one more sin that is piled onto Jesus at the cross. (I do not know if Jesus feels a new pain every time we sin, but it seems plausible to me that he would, or at least did at the cross.)
Remember what Jesus said to the woman caught in prostitution:
John 8:10 and 11
When Jesus had lifted up himself, and saw none but the woman, he said unto her, Woman, where are those thine accusers? hath no man condemned thee? She said, No man, And Jesus said unto her, Neither do I condemn thee: go, and sin no more.
Jesus told her to go and sin no more, he did not add "if you do after I go to the cross; it won't matter anymore."
By these Scriptures. We should understand that even though Jesus paid our sin debt in full. That does not mean the Law has been done away with. Thou shalt not commit adultery is just as cogent now as it was when Moses told the people of Israel coming out of Egypt.
There is much truth in the old saying that no sin goes unpunished.