Australia invests in immigrants alone.

Getting married before emigration is cheaper and faster, because you can apply for two people at a time. If you immigrate before you get married, you have to submit two immigration applications, and in order to check the marriage immigrants, you need to submit a lot of evidence to prove that you are really married, which inevitably adds a lot of trouble and is completely unnecessary. Immigration is a laborious task, so don't artificially increase the burden.