Mark is a theologian who used to post over at Shepherd's Fellowship, he also wrote the book Redemptive Divorce. He DOES (sorry, I said earlier he didn't) speak about the verse you referred to over at teampyro; I hope this helps explain it some for you. Mark is also the son-in-law of the well known preacher, Chuck Swindoll.
The Sin of Lust
Earlier in His ministry, Jesus commented on the teaching of the rabbis concerning the Law and then offered His clarification as the divine Author. “Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill” (Matt. 5:17). He then took the opportunity to breathe new life into some Old Testament laws in order to correct the teaching of the Pharisees. Note how He presented His lessons:
* “You have heard . . .” (v. 21), “But I say to you . . .” (v. 22), followed by teaching on murder and resentment.
* “You have heard . . .” (v. 27), “But I say to you . . .” (v. 28), followed by teaching on adultery and lust.
* “It was said . . .” (v. 31), “But I say to you . . .” (v. 32), followed by teaching on divorce and fidelity.
* “You have heard . . .” (v. 33), “But I say to you . . .” (v. 34), followed by teaching on vows and integrity.
* “You have heard . . .” (v. 38), “But I say to you . . .” (v. 39), followed by teaching on justice and kindness.
* “You have heard . . .” (v. 43), “But I say to you . . .” (v. 44), followed by teaching on fair play and grace.
In each case, Jesus extended the application of the Law given through Moses to include what the rabbis had omitted. Furthermore, He amplified the divine revelation in the Old Testament to reveal the full measure of God’s standards. Not only must we refrain from murder, but we must also avoid hatred. Not only is adultery an abomination, so is lust.
While the laws of the Old Testament reflect God’s righteous character, they were primarily intended to regulate the public affairs of a nation, much like the laws of our own government. But we generally understand that a person must be more than merely law-abiding to be considered moral. Obedience to the law is a minimum standard. The rabbis in Jesus’ day not only reduced righteousness to mere obedience to the Law, but they also played clever word games with Scripture to lower the standard even further! They lowered the standard of righteousness in order to call themselves righteous.
When Jesus equated lust with adultery, He was not suggesting the men apply the Law accordingly. It was to point out their hypocrisy. It was to confront the wayward rabbis for lowering the standard of righteousness. It was to convict the self-righteous of their sin.
Correlating the Teaching of Christ on Lust and Divorce
As we examine the teaching of Christ on the Law, we must apply it in the New Covenant sense rather than under the Old Covenant. If we are to apply the Old Covenant strictly (Lust = Adultery = Grounds for Divorce), we must do so consistently: Lust = Adultery = Grounds for STONING!)
When Jesus confronted the rabbis, His purpose was to show that no one can be called righteous, even those who are not guilty of murder or adultery. Our hearts are thoroughly polluted with sin; even our thoughts make us guilty. Therefore, ALL are guilty before God and ALL need His grace.
If we apply the teaching of Matthew 5:27–28 in the same spirit Jesus gave it, then equating lust with adultery is the kind of confrontation needed by men viewing pornography. They want to rationalize their sin by stating it doesn’t involve actual contact with another. We must help them raise the standard of righteousness, not lower it if they expect God to bless their marriages and hear their prayers (1 Peter 3:7)
Mark's blog is http://www.redemptiveheart.com/. I forgot to mention this earlier.