Marriage is a contract.

I doubt there is any argument there. A couple sign a legal contract saying they will live together and possibly procreate.

Nowadays many couples decide not to bother with this legal commitment [and expense] and just move in together and presumably enjoy mutual sex..

Again, I doubt there is any argument on this state of affairs.

So if you boil it down to the bare bones, a modern “marriage” is where one party moves in with the other and they live in a shared home with sex as a bonus. This arrangement is at the heart of the impending referendum where the term “marriage” is being replaced with “durable relationship”, There is no definition for “durable relationship” so it could easily cover the “sex for accommodation” arrangement.

But they want to make that illegal.

So technically every non-married couple would be breaking the law, while at the same time their situation is copper-fastened under the constitution?

Just a thought.