As you will see when you read Matthew 19:16-21, Jesus was quoting the commandment
not to commit adultery. By this Jesus gave his approval of the Old Testament definition of
adultery since that is the only place in the Bible that a definition of adultery can
be found. Amazing as it may seem, there is no definition of adultery given in the New
Testament and we must go to the Old Testament for that. The definition is found in the
following verses: Leviticus 20:10, Proverbs
6:32-34, Jeremiah 29:23, Ezekiel
According to the verses just given, adultery is only defined as an act that takes place between a married woman and a man who is not her husband. An act between a married man and an unmarried woman is not adultery! Why would you want to call it adultery if it is not? It however is a sin because it is a premarital act. It is the very same sin whether the man is married or not. A premarital act is always a sin. However, if a man marries a woman in addition to the wife he already has he has not sinned. (See 1 Corinthians 7:28, Deuteronomy 21:15&16) Why would you want to say that having more than one wife is a sin if it is not. There is no verse in the entire bible that gives even the slightest hint that having more than one wife is a sin.
Now that you know the biblical definition of adultery, the one Jesus accepted, you can finally understand what Jesus was saying when he said if you look upon a woman with lust you have already committed adultery in your heart. Jesus was talking about a married woman! The key word in the verse is lust. Which according to Strong's Greek concordance means to covet. You can only covet that which belongs to another. Obviously if the act with an unmarried woman is not adultery then the act in your mind with an unmarried woman is not adultery. If the "act" in your mind is for entertainment purposes then yes it is a sin. If the woman is someone who is truly available for marriage and the thought is one of normal attraction and you intend to seek her as your wife then it is not a sin. If you are merely entertaining thoughts about random women then it is most certainly the sin of "fornication in your heart." Do not however think that since you have committed a sin in your heart that you may as well commit it outright. That is folly and a much greater and deliberate sin.
16 And, behold, one came and said unto Him, Good Master, what thing shall I do,
that I may have eternal life?
17 And He said unto him, Why callest thou Me good? there is none good but one, that is, God: [Author's note: Jesus made clear in prior verses in the very same book by Matthew that He was indeed God: Matthew 16:16 And Simon Peter answered and said, Thou art the Christ, the Son of the living God. 17And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven. John 1:1 contrasted with John 1:14 even more clearly illustrates that Jesus is God: 1 In the beginning was the Word, and the Word was with God, and the Word was God. 14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.]
18 He saith unto him, Which? Jesus said, Thou shalt do no murder, Thou shalt not commit adultery, Thou shalt not steal, Thou shalt not bear false witness,
19 Honour thy father and thy mother: and, Thou shalt love thy neighbour as thyself.
20 The young man saith unto him, All these things have I kept from my youth up: what lack I yet?
21 Jesus said unto him, If thou wilt be perfect, go and sell that thou hast, and give to the poor, and thou shalt have treasure in heaven: and come and follow me.