Understanding why women cheat may help you make changes in your relationship that will help prevent this from ever happening to you.
Have you cheated on your partner? If you have and she finds out, she may think that she is entitled to do the same to you. Revenge cheating might not seem like the best rationalization for infidelity, but you opened the door to this the moment you walked into the arms of another woman. The bottom line is that if you are cheating, she is more likely to start cheating as well.
Another possilbe reason why women cheat is if she is feeling taken for granted. Yes, relationships change over time. You may not shower her with roses as much as when you first met, but everyone wants to feel special. If you aren't taking the time and making the effort to make her feel as though she is incredibly special, she may go find someone who will.
A woman might also cheat if the two of you no longer have a lot in common. For example, if you want to spend all of your free time playing softball with the guys and she wants to spend all of her free time playing coed volleyball, not only will you not be spending any time with each other, she will be meeting other potential partners who have more in common with her than do you.
Some women have personal issues that make it seem impossible for them to be in a healthy relationship. For example, she may cheat simply for the thrill of cheating. Or, she may have self-esteem issues where she needs someone new all of the time to make her feel beautiful or valued. Or, she may also just be a horrible human being. It may be time for you to examine what attracted you to her in the first place so that you can avoid making the same type of commitment in the future.
Why do men lie about cheating, especially once they are either deep into the cheating or have been caught? If you take a step back, you can learn to at least understand why they lie when they do.