What do you think of women who are "the other woman"?

I used to think women who was the other women were uncaring, stupid or selfish until I almost did the same thing. I fell in love with someone who had a long term girlfriend, was never stupid enough to believe that he loved me the way he loved her though and I felt very sorry for her but then I realised she was more stupid than I was cause she stayed by him after she knew what he was. I hated him but I at the same time I loved him. I will never go there again with anyone.