This seemingly became a bit more complicated when the Crows, then Port joined the AFL.
There are some who just went for the club that their fave SANFL player went to, others that followed the club because of their Dad/other, etc, etc.
So you’d initially think that a Port SANFL supporter would just support Port AFL, but there’s Port SANFL supporters who follow West Coast, Sydney, Richmond or even the Crows.

Makes sense if you followed one of those clubs before Port entered the AFL.
Similarly, there are Glenelg supporters who follow Port AFL, Carlton, Geelong, etc.
There’s probably a similar thread for why your SANFL club, but we’ll leave that for another day.
So, what’s your story?