Usually people date because they want to work towards a romantic/sexual relationship (and not towards friendship). They might start off as friends, but generally they are trying to move away from friendship to another kind of relationship. And they might break up and still be friends, but it's not exactly true or false that they are friends <i>while</i> they are dating.
It's not easy to give a black and white answer because the whole point of dating is to be something other than strictly friends, but dating is a process and people who are dating haven't gotten to that other stage yet, so they could still be just friends in a way (if they were friends before they started dating).