This is pure speculation, but while Bella is not interested in doing anything with Burke now, that does not mean that she didn't slip up with him three years ago.
I guess some people would be turned off of Bella, if that turned out to be the case. For me, I'm not too concerned over what she might have done back then. What she is now, is much more important. Of course, none of us knows yet what she was then OR what she is now.
Again though, this is all guessing on my part. Sooner or later DrPinkCake will reveal all.