The thing is, ComeOnGordon, most - if not all - people lie. They lie about different things, some minor, some not. Women (and men) don't, from my own experience as an OW, make a beeline for somebody partnered. What the partnered person is thinking, I don't know because I wasn't at the time.
Relationships develop between people all the time and I think it's a truism that when they are perfectly innocent, and the two involved are cognisant of that fact, there's no reason to stop the relationship because it is above board. If it develops, it's often done by miniscule degrees - like the frog in boiling water analogy - and by the time it is something that should be stopped, the pair are too enmeshed and it's too late. Some do stop, but for most, if not all, I would say that there's a fair amount of life assessment going on as they realise that their horizon is wider than they thought it was.
For some couples, having their eyes opened to their relationship being under threat is an opportunity to take stock and pull up the drawbridge, and for others it's an opportunity to end it. And it's the right thing to do.
Just as OW can't know what your husband is like when he's in a temper or leaving his pants on the floor, the partner at home can't now what their husband is like when he's with the OW. I personally believe that part of the danger is that in a long relationship, the lustre is replaced by burnish. That has a value of its own but for some, the need for lustre will prevail.
For OW (and presumably OM), their relationship is one of euphoria quickly followed by the dread and knowledge that they've 'lost control' of their feelings, which they have. It's miserable for all parties really. I've been on both sides. I would never choose to be an OW again and the 'danger points' I know well and they protect me from ever falling into that again.
I don't know if self-esteem is the key thing - or whether it's Daddy-issues - or something else. There are probably as many reasons as there are people.