@Ian *********
That is all I needed to marry my Thai wife. Free to marry certificate from the Australian Embassy. And my Aus passport (back then in 2003, I was on 30 days tourist exempt).
Yes, to marry, we needed 2 witneses, the agency sent two of them. 30minutes and the official marriage was registered (Bangkok).
There was a toned down ceremony, 5 minutes, ended with "Nothing other than death can separate you".
The copy or certified copy of that marriage was needed in Japan and even in Thai (for the Yellow Book).