Having said that, I do think blame can be a complicated issue.
I'm going to admit that I have slept with a married man. Not an affair, a one night stand and his wife doesn't know. But still. Technically having to put my hand up to being an OW, I guess.
For a long time I condemned myself as being a truly terrible person. I still (obviously!) think it was an awful thing to do but here's my current assessment of the blame and exoneration of all three parties. Feel free to comment and disagree as robustly as you choose.
The man:
Blame - he is married and (at the time) had a few children with another on the way. He came back to my house late at night after the pubs shut, drunk. He came on to me, kissed me, told me I was wonderful etc. He guided me upstairs. He laughed it off in the morning but continued to get flirty and try to kiss me when drunk for years.
Exoneration - he has bipolar disorder. his marriage is stable but not very active, if you see what I mean.
Me:
Blame - I knew he was married. I was drunk. I was half in love with him and just went along with it.
Exoneration - I have anorexia and a strong conviction that I am ugly. I am always stunned if someone shows any interest in me at all. The insecure part of me responds to it regardless of my own feelings and the sado-masochistic part of me wants to fuck up my life even further.
The wife:
Blame - she used to refuse to leave the house to do anything social with him, ever. She wanted to stay with the children all the time. She didn't like physical contact much.
Exoneration - she did nothing wrong!
In summary - I think more of the blame was his than mine (and certainly hers!) but I do think I was partly to blame too (I don't think she was).