Matthew 5:27-28 (King James Version)
"Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart."
The dilemma here is that there is very distinct misinterpretation of this verse; in the form of both eisegesis and perhaps not scrutinizing the verse enough.
To clarify, the word "adultery" refers to sex out of wedlock -- whether it's a wife cheating on her husband, a husband cheating on his wife, the "other woman" or the "other man". Contextually speaking, it's evident that Jesus is referring to the following order (one of the Ten Commandments) in the Old Testament:
Exodus 20:17 (King James Version)
"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."
What can be garnered from understanding this is the general message the Messiah was trying to get across, which is reinforcement of the commandment against coveting your neighbor's wife. By understanding this, we can deduce that Jesus was not at all referring to unmarried folks, but those already in wedlock.
To get a better understanding of this, lets look at the Greek Koine word for "lust", which is the source of the heat. The word, in question, is "epithumeo" -- which is best defined as a strong, passionate, consuming desire for something that is forbidden. Think of it like a heroin-addict trying to go clean -- maybe a bit less-so.
So, to give you a better lens of what Jesus was trying to say through a modern lens:
"In the Old Testament, you were told not to covet your neighbor's wife. However, sin is of the mind and being consumed with the desire to have sex with your neighbor's wife is no different."
Alternatively or maybe simultaneously, it could mean:
"In the Old Testament, you were told not to commit adultery. However, sin is of the mind and being consumed with the desire to have sex with someone other than your wife is just as bad as if you actually do it."
Either way, it's evident by the context that this is not an admonishment of lust, in of itself, but adultery.