What is this puritanical obsession with incest among the anti-Trump brigade? Trump is crass and even vulgar at times. That's the way he speaks. Nothing new here. Most people accept it or not already. There is no indication at all that he has had inappropriate relations with his daughter, as much as some people want to believe that.
If Clinton said it, it would come off as creepy because of his track record with women. But he would get a pass as well. If Obama said it, it would be so out of character people would think he is on drugs.
Context. It matters.
I don't normally watch the debates, but this should be a good one. The worst thing that can happen to Trump is that Hillary fails so spectacularly (or collapses) that she is replaced by another candidate. Almost anyone would make a stronger opponent than Clinton.
Either way, it's all just popcorn and entertainment on the Titanic.