I probably was.

There was also no option for "in law" options, which I thought might be important to know.

I am quite enjoying people complain about providing all this information to "the grubberment"... on Facebook. With all their details filled in and profile not private in any way.

You can let them fill in the section for themselves if you trust each other not to look at each other's parts of the form, or they can click the button on the census page to create a new form.

They say that if they get two forms for the same person they will keep the most complete one

This is pretty good for privacy

It was a bit old fashioned on marriage though, you're only married for census purposes if it's registered, but there's no need to register a marriage now, you're treated the same everywhere it matters (a defacto spouse is treated like a spouse for medical purposes, for example)

I wonder why they don't want to know how many of the marriages are registered and how many are not.

So reading the ABS website they could have contacted the ABS to ask for a separate census form or should have filled it out prior to the 11th of August if they knew where they would be on the 11th.

You can find the information here.