The reasons may be varied, but some of them may include a lack of a moral compass, or caring that this may hurt others. Indeed, it may also hurt the mistress most of all. Other reasons could include a low self-esteem, e.g., settling for less than she could have with someone truly available, or perhaps an over-riding priority , a selfish one, that being "in love" over-rides all other considerations. Some people blindly believe such promises as "I'm going to leave her, but I can't right now.", etc. , when it only serves to keep the mistress involved without having to upset or end the marriage. Another possibility is a lack of awareness that, if the husband is willing to cheat on his spouse, he could indeed be willing and able to cheat on the mistress, then or in the future, as the best predictor of future behavior is past behavior. I do think that if someone is vulnerable and willing to engage with a married man to begin with, they may be either foolish, stupid, uncaring, or so needy that they have blinders on. And, there are some who simply may feel safer in the relationship, since there are fewer demands upon them. And there are also those who are exploitive, who engage in such a relationship for the gifts and financial advantages that may be offered. Lastly, there may be women who are very unhappy in their own marriages and who are willing to go outside the relationship to seek fulfillment, while still remaining married. So, all kinds. Does this answer your question? I'