Can you give a general outline of what the "spirit" of the 6th Commandment is?

My understanding of it is don't have sexual relations with someone who is not your actual spouse because that contradicts the absolute fidelity of God to His people as well as the original unity of Adam and Eve in the garden prior to the Fall. 

Am I  getting that wrong? Help, please! The "spirit" of a law can be so...how to say it...murky, hazy...grayish at times!