Some single women date married men to avoid commitment. Some date married men because they at least know a married guy is capable of comitting. Some just think that the A makes them special. Some are bored. Some get caught up the drama.
There really are lots of reasons.
I will add to DCK's assessment and say some absolutely have no morals and would do anybody any time, regardless of whether they are married.
There's all kinds of reasons, but if they cry when they get dumped, more often than not, I believe it is because they are hurting. (Not that I care!)
One thing I truly have come to believe about a lot of people is that they often lack empathy, especially in situations where they are attempting to take something they want from someone else. They feel entitled to that something, and when they don't get it, or lose it their feelings are real, and they don't even consider the person whom they were "taking something" from. Their own feelings count, but nobody else's do.
The whore from my situation was actually married herself, but I believe she thought my H would be a step up for her, from her own spouse. She had an A with him when she was much younger, and he was married to his first wife. She was married that time also, to someone else, not that my H ever bothered to tell me that. I found out through my own investigations when I became more curious about what their history was. To this day, my H can't tell me where this supposed H was. I'm guessing he was in the military and she was staying with her parents. He did tell me her parents encouraged the A!
If I make guesses he will just say, "yes, I think that is right." He claims not to remember. It was 25 years prior to the A that happened in our M, and he does not have a good memory but you would think he would remember THAT.
But knowing how my H is, and with what I have figured out about the Whore, I think she likely highly romanticized him based partly on the fact he had an A with her when she was young. She probably thought he was "noble" for dumping her and going back to his wife.
During our M, she went after him after the deaths of his mother and DD, when he was out of his mind with grief. This is not to let my H off the hook for his part, but it was 7 years ago and to this day, I feel it was at least as much, if not more her fault this happened. I do not believe she could have been "just anyone." She was a very specific person who decided to use a very specific set of events to better her chances, and it worked, up to a point. Of course she got dumped. Again. And of course she cried. In fact I got to hear her blubbering and bawling on the phone when she attempted to leave voicemails for my H after she'd been dumped.
I'll bet she thought he was "noble" during our M too because he would not allow himself to be alone with her for long enough for her to try to get him involved in sex. Even though they said ILY to each other and verbalized plans to run off together, I'm fairly certain in her mind this reinforced in her what a good guy he was, and her life would be so wonderful if she won him (she most definitely planned to dump her H if mine would be her new meal ticket).
She also saw him as a very successful business man and "admired" him. I'm sure she was able to tell herself that I didn't appreciate him and didn't deserve him.
Even though she was (is still) married, I mostly put her with the group of usually single OWs who actually go after what they believe is a committed family man, because they think that is what they are going to get, if they get him away from his wife. My H would talk about the trips we take, the things we do together and certain women seem to really show signs they would go after that because they are jealous of the life we live. It starts seemingly innocent with them just comparing their own man unfavorably, and saying "how lucky your wife is" that you do this and that and the other thing. I hope she appreciates that! And then they would cleverly look for the bad days the man might have had at home, or any sign they might have an inroad to pursue...and they do it!