Many women find themselves attracted to a guy that is already in a relationship. It is not uncommon for people to want what they can't have. That is why guys that are taken seem more attractive than those who are not. Therefore, some women will do their best to catch the interest of a taken man, just to see if they can. It makes a woman feel good if she can turn a guy's head, especially one that already has a girlfriend or wife. It is considered a stroke to their ego because it means they still look good enough to catch a man's interest, relationship or no relationship.
In a woman's mind, a woman wants him so he must be worth of a relationship. Therefore, it is easy for her to view him as a trophy. Women can be very competitive by nature. Therefore, they will compete to win another woman's trophy over. It makes a woman feel accomplished and better about herself is she can successfully steal another woman's man.
There is obviously something special about a man if he is in a relationship with another woman. Perhaps it means he is good in bed or he is financially secure. Either way, a woman's interest is typically peaked by his relationship status.
As previously mentioned, women are competitive. A man that is in a relationship is challenging to obtain. It can really boost a woman's self-confidence to obtain the interest of a man that is already in a committed relationship with another woman. It makes a woman feel beautiful and good about whom she is. She knows that she must look good to capture the taken man's interest, especially if he decides to keep contact with her and flirt with her behind his woman's back. This means that he is obviously more interested in her than his own girlfriend is.
Women love the chase sometimes just as much as a man does. They like something that is hard to get, including a man that is in a relationship. It makes it challenging for them. They are willing to go the distance to see just how good they are to make a man turn away from his current love interest. If the woman is unsuccessful at capturing the interest of the other man, it means that she no longer has the ability to turn a man's head and this can be a serious hurt to the woman's ego.