It is not certain that it will out him, but it will certainly get some people wondering. There was a "youth parliament" in Melbourne last year that debated, amongst other things, same-sex marriage. The topic can be tackled as a social justice issue and there are (straight) people who do so on that basis. However, it is a risk to use this as a topic for a class paper and it should be acknowledged. Only Justin can make that call.
One of reasons I suggested he research Warren Entsch is because 1. he's straight, 2. he's a federal politican and 3. he believes passionately in gay rights. He's also from one of the most conservative parts of the country.
An email to Warren Entsch may get a response in time, as could an email to Justice Michael Kirby (an openly gay judge on Australia's High Court). Other public figures he could email would include Senator Bob Brown (the leader of the federal Greens Party, also gay) and Dr. Kerryn Phelps (former president of the Australian Medical Association, also gay).
I can't guarantee any of these will respond, and I'm not sure where to get Dr. Phelps's email address from, but the email addresses of the rest are public information (check out the Australian Government web site).