You know, I have to say that I agree with both sides: those who would protect their daughter from the co-ed party and those who would be willing to host one. There's nothing wrong with being a parent that will allow boys and girls to have an all-night party, so long as rules are clearly established and you're willing to supervise the entire time. That said, I think it would be silly to be offended if a parent refused to let their daughter come. They probably don't know you very well, and they probably don't know the BOYS very well. They have the right to be concerned and protective.
My answer to this problem would probably be to tell my daughter NO, she can't go to a co-ed party when I don't know everyone involved. But then I would offer to let her throw her own co-ed party at my house, under my strict supervision. I attended a few such events as a kid, only ten years ago, and nothing immoral happened at any of them. But that doesn't mean I trust the neighbor kids as far as I can throw them...