This is no defense at all. You can't say "Yes, I knew there was great likelihood it was wrong, but I didn't know it for a fact that it was wrong, so I was actually in the right." It's a bit like punching somebody in the face and then saying, "Well, how was I supposed to know he didn't want to be punched? Some people like to be punched!"

If you can circumvent copyright just by not caring whether you have a right to copy, that's essentially the same thing as copyright not existing at all.