It depends- there's a difference between a woman who meets a guy at a bar or online, they date, get to know each other a bit, but he fails to mention the little inconvenient fact that he's married. There are a lot of men like this (with some variations (the marriage is over, we just live together because we can't afford to sell/the children/etc - it's an open marriage that I've failed to discuss with my wife or let her know it's open for me.) It can be devastating to discover you are the OW when you had no idea, especially if you've started to fall in love.
It's a different situation setting after someone she knows to be married but she wants the challenge of seducing him, or the money and lifestyle.
And there will be people who meet at work or somewhere, get on very well, make an effort not to start anything but realise they're in love.
Some will have low self esteem, feel it's no threat to the marriage, but he's the only person who has looked at her, all she deserves. Knowing it's wrong just confirms she's the terrible person she already believes herself to be.
Or she's in a bad marriage, feels she can't leave because of the children, money, (hasn't discovered MN,) whatever - and an affair gives a glimmer of light to her days, she feels wanted and desired if only for a few minutes.
Most people who are aware the affair partner is marriage will probably tell themselves all sorts of reasons to justify it and many will feel guilt. Of course they know they shouldn't do it, but they'll find reasons to justify it to themselves and they'll feel guilt.