So for example, lets say a single girl sends some guy a picture of herself naked. Is it wrong/sinful of her?
Please keep in mind my earlier point in this thread that Matthew 5:28 is not about single women it's only about married women.
Also keep in mind that this woman is not being forced to do this, she enjoys it and wants the guy to enjoy seeing it too.
No, she's not being forced to do so. She's just sinning herself, by lusting after the man she sending the picture to. Just as in the law here. If you are driving the car that drove away from a bank robbery, yet you never went into the building, and in the process of the robbery someone was killed. You can also be charged with murder. No one is innocent from sin.
Now to Matthew 5:28.
Context, is very important.
First Matthew 5 is the Sermon on the Mount. Christ is speaking to a crowd of people and he uses stories, true stories and parables to tell the crowd the Gospel that God wants him to spread. Because it was easy for them to understand.
Plus, during the Sermon on the Mount Christ was showing that the law didn't go away, because He came to be. He enhanced them and backed them up.
In verse 21, says this:
21. You have heard that it was said by them of old time, You shall not kill; and whosever shall kill shall be In danger of judgment:
22. But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council; but whosoever shall say, You fool, shall be in danger of hell fire.
Christ hits on the 6th commandment of Thou shalt not Kill. It's pretty plain and simple what He is saying. But it also shows us that not only does he see what we do.
God sees the thought-life. He weighs our motives and judges the intent of the hearts: "Whoever hates his brother is a murderer" (1 John 3:15). Right after those, he explains that if we have brother has something against us, we are to step away from our prayers, ur with our brother and then go back and make our offering of prayer. He uses a gift as the example.
He then tells us to agree with our adversaries, because if not, he could take us to court and we could be judged by the courts for a wrong doing.
Basically we should ask for forgiveness if we bring harm to someone.
Then you get to verse 27:
You have heard that it was said by the of old time, You shall not commit adultery:
28. But I say to you, That whosoever looks on a woman to lust after her has committed adultery with her already in his heart.
Then he continues about what we should do to keep ourselves holy. He doesn't mean that we are to physically hurt ourselves. It's called a hyperbole...we use it all the time.
So verse 28 isn't about one certain woman. It's any woman. Married or not married. We are not to look upon a married woman or even a single woman and lust over her.
You also need to understand that there is the 10th commandment, Thou shalt not covet.
Coveting covers a huge area. Adultery falls under it. that's wanting that person your lusting after even though you can't have her. Lust of the eyes, falls under it. Wanting what others have. But most importantly.
God knows what's in the heart: "For God will bring every work into judgment, including every secret thing, whether good or evil" (Ecclesiastes 12:14). "But after your hardness and impenitent heart you treasure up to yourself wrath against the day of wrath and revelation of the righteous judgment of God, who will render to every man according to his deeds" Romans 2:5,6
Mans heart is sinful. Jeremiah 17:9 affirms the condition of man's heart: "The heart is deceitful above all things, and desperately wicked: who can know it?" Verse 10 then warns us that God not only knows the secret things of the heart but will reward us accordingly: "I, the Lord, search the heart, I try the reigns, even to give every man according to his ways, and according to the fruit of his doings.
There's more:
Mark 7:20-23
20 And he said, That which comes out of the man, that defiles the man.
21. For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders.
22. Thefts, covetousness, wickedness, deceit, lasciviousness, and evil eye, blasphemy, pride, foolishness:
23. All these evil things come from within, and defile the man.
So, we are sinful. If you have looked a woman with lust you have committed adultery in the eyes of God. Because God sees our thought life and He knows what's in the heart.