Odds are incredibly high the kid lives in a straight household. If you just say you are married, the child will understand that your family unit is like their family unit. Without some kind of prompting, how many kids have ever asked a teacher if they are straight? It is assumed.
Assuming a gay Florida teacher was even willing to risk their job by placing a picture of their partner on their desk. Saying they are married in that situation would likely invite questions. And that is okay, kids should be able to ask questions!
The default is transparent and socially accepted. All other configurations are visible, divergent, and sexualized.