Human beings are competitive by nature. People tend to value things that require effort or a challenge to obtain. Those things which are perceived to be in abundance are generally thought of to be less valuable. (dirt, water, salt...etc)
According to many articles and media outlets a "good man" is hard to find. Therefore these types of women that observe a so called "good man" are likely to try to see if there is a chance he is not happy or they can possibly get him for themselves. As opposed to believeing there are other men available just (like him). Nothing is more challenging than taking someone from another person especially if the couple appears to be happy.
Last but not least the (lazy) or (frustration) factor plays a part. These types of women may be tired of the "tricky world of dating" and frustrated with the whole process of finding a "suitable man". They realize they are working with the kind of man they want but he is married, or there is "good man" who is married that lives across the street from them, or they know a 'good man" who is married to one of their acquaintances. Sometimes these women are actual friends of the wives.
In many instances the wives have been (bragging) about how good the man is in bed or how much he adores her. This does nothing but make her man seem all the more valueable in these women's eyes!