OP - i was in your position. In my experience, it was a desperate attempt to leave an extremely unhappy, EA marriage.
Of course I knew it was wrong. By that stage I was an unconfident wreck. Couldn't leave (a bit like telling someone who is depressed to 'snap out of it' ) - despite continually trying. Ex H threatened suicide / sneered / said he'd take the children etc if I did. When your confidence is rock bottom, you believe all of that.
On top of that i had immense guilt about the affair. Immense, to the point I felt worthless.
I'd never have posted on here to get even more of a kicking - I think it would have finished me off. As a previous poster said, it is rarely without huge amounts of guilt!
I did a terrible thing to my ExH and will pay the price of guilt for the rest of my life. He discovered the affair, and my world (and his) collapsed. I would (and did!) have done anything to make it better - I became a prisoner in the house while he sneered and told everyone. I slept with him on demand because "you don't get to call any of the shots from now on" and was allowed out for work only.
Fortunately 5 years on, we are friends for the same of the children, and ok - but not together. He has a new - ish partner and I am with the OM and happy. During our affair, his wife left him for someone else - so that minimised the guilt from that end.
I guess what I'm trying to say is that it isn't as simple as "all OW are evil" - there is often a lot behind the scenes.
That doesn't make it right - far from it - but if you take anything from this OP, it would be that the guilt never, ever leaves. 4 rounds of counselling and 5 years later - it's still there!
Take care.