Sin is a heart-problem. Circumstances, environment, heritage, &c may contribute to the liklihood that a person will commit a given sin, but the heart of the human problem is the problem of the human heart.
In just a few verses, Jesus gives some very convicting teaching - adultery starts in the heart. As with murder, adultery begins long before the full-blown sin occurs. Listen in to hear what Jesus has to say about lust and adultery.