“You have heard that it was said, ‘You shall not commit adultery.’ But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart. ~ Matthew 5: 27-28
Even if no one has noticed you gazing at her... your heart doesn't lie. Be careful guys. Never allow a glance turn into a gaze.
Jesus clearly teaches us that purity begins in our heart and mind. You can appreciate beauty - just don't idolize it. If you catch yourself staring a little too long... turn to look at God and switch that that pathetic thought into a powerful prayer for object in your eye.