I have just got back from 3months in Australia. I spent most of my time staying in hostels, most of the time I stayed in mixed dorms. I had no problems at all, to be honest the best part of my trip was a week I spent with a great bunch of guys I shared a room with. I was travelling on my own and I never felt uncomfortable or unsafe, so my experience of staying in a mixed dorm is all good! (other than some smell a little more than girls only dorms!!)