a married woman does not get the attention she wants from her husband, so you offer your friendship, and promise yourself that it will just be friendship, and not cross the line. But, because you are investing time, emotions, sharing secrets, giving advices, you can cross the line even against your best intentions. You just can't have one bite of potato chips, and not go for another bite. and perhaps another? till it's consumed. Human nature. She needs attention, you give attention. She needs emotional support, you give emotional support. If that's your definition of friendship, it is also the formula for an emotional affair. (maybe physical, later)

but if you are two complete people, happy with each of your own lives, and fulfilled with each of your own marriages, then friendship is safe and can be wholesome.