Since his son Nathan came out 12 years ago, Thomas has been a vocal advocate for same-sex marriage and other LGBTI causes.
And now he's spreading the message in an extremely Australian way: by decorating his ute.
Thomas told BuzzFeed News he "grew up homophobic" and spent a lot of time in settings that were hostile to gay people.
"I spent nine years in the army, I'm a Vietnam veteran, I was a plumbing contractor," he said. "I was conditioned to be homophobic, and then one day my son comes out to me.
"I had to ask myself the question – what is it about gays that I didn't like? After that, I decided it was unfounded fear, ignorance and prejudice. Then I came to the view that my son wasn't equal in law and that really got up my nose, so I became a very strong advocate for marriage equality.
"I'd had this talk with some people and I said, 'Every man and his dog supports marriage equality'. And I thought, I'll put that on my ute!"
The ute, which is a few months old, received the equality makeover last week. Thomas paid for the signs himself.
Thomas said he spends a lot of time driving around Sydney, so plenty of people will get to see his rainbow flavoured vehicle.
Later this month he will take the ute on a round trip to Melbourne, Mount Gambia and the Riverina region to talk to people about same-sex marriage.
Thomas said he doesn't understand why we can't legalise same-sex marriage in Australia.
"To me it's a civil rights issue. It's got nothing to do with religion, nothing to do with gay people having babies. In the 12 years I've been advocating for this, I'm yet to hear a reasonable argument against it."
The Australian parliament is at a political impasse over same-sex marriage, despite majority support among federal politicians and the Australian people.
The government policy is to hold a national vote on the issue before it can be legislated, but that policy was defeated in Australia's Senate by politicians who argued it should be subject to a regular vote in the parliament.
Unlike Ireland, Australia does not need to pass a national vote before same-sex marriage can be legalised.
Lane Sainty is a reporter for BuzzFeed News and is based in Sydney, Australia.
Contact Lane Sainty at firstname.lastname@example.org.