It teaches that we must not look lustfully at each other and nor should we behave in such a manner as to entice others to lust after us. I made a covenant with my eyes not to look with lust upon a girl. While the above verses refer to men lusting after women, all women know that it can very easily be reversed to apply to themselves as well. Men may be more easily visually stimulated than women, but women can be just as vulnerable to sexual fantasy in the emotional realm. Both are sin in God's eyes, and both can be brought into subjection by controlling our thoughts through Christ's power.
All sexual immorality begins with a thought.
A lustful thought not taken captive, will eventually lead to other perversions, because sin reproduces itself in increasingly greater measures. If we do not deal with our evil thoughts, they will take root in our heart. It is for this reason that God is so concerned with our thought life. Our loving God never demands from us what is impossible for us to do. We are so weak within ourselves that it may seem impossible, but He will equip us with His holy power to overcome any sin, if we ask in faith.