When I was a youth pastor, there was one question I could always count on whenever the topic was sexual purity. "How far is too far?" The question wasn't a request for intellectual information; it was all about knowing where the line was - so it could be walked right up to.
The Pharisees would have asked the same question. They wanted to define the boundaries for their behavior so they could know what lines not to cross - but they felt their hearts, attitudes, inclinations, desires, etc. didn't really matter. In His exposition on the law during the Sermon on the Mount as recounted in Matthew 5, Jesus reveals that the truth is deeper - and harder.
The law forbids certain behaviors, but what God is really interested in is the intention of our hearts. That is why "adultery" isn't just an action; it's also a lusting of the eyes. The prohibition against "murder" isn't just about a physical killing; God doesn't want us raging at one another.
Christians are called to grow beyond a juvenile understanding of the law as a list of don'ts. The Law reflects God's desires for His people - and it's more than just a matter of conforming our outward behavior. He wants us to become the kind of people who will naturally operate within His will. That's the Spirit of the Law.