A woman is fully capable of being faithful. It is not so much in their nature, although it is of course possible, to be drawn to someone else when they are already in a relationship.
Temptation is not her weakness. That's one of the key issues with men... wondering eyes will often lead to a compromise of values and succumbing to physical lusts.
What most women want is Love, Attention, Honesty, and Loyalty. These things, admittedly by most men, are emotional needs that men either struggle with expressing, do not care to express, or, are simply "clueless" to the fact that they exist and are essential to fulfillment, joy and happiness to most women.
Love her! Love is best expressed through giving and sacrifice. Give to her, your time, your things, but most importantly your heart. Share yourself with her. Sacrifice yourself, put her desires above yours, sacrifices your wants fans desires to see that hers are fulfilled
Pay attention to her. This shows her that you value her and that her opinions and ideas are respected and that they are important to you. She needs this.
Be honest with her. Even when the truth may be unpleasant, tell her the truth. Her intuition will alert her if you are not, and she will sense that something is not right. If you are no longer wanting to be with her then tell her. Be honest. Don't cheat
Be loyal. Be faithful. Be monogamous! Trust her and put her first.
Remember that before she "cheats" she will wine. cry, and complain about all the things that make her unhappy. Listen to her, open up your heart to what she's saying. If you don't understand, ask her to explain it so that you are clear as to what she's feeling. You must know her on this level. This IS intimacy! and sadly many couples lose their intimacy as their relationships move through time.
These are all warnings before the destruction of the relationship comes. So pay attention to your woman, and never cause her to feel like she's alone and single or thats how she will act. Like a single woman, and you will lose her forever.
