Hey there @Cyrinus -- to rotate furniture in the House Decorator, all you have to do is left-click or tap it.

Of course you probably already know that this only works on some furniture, like you mentioned couches, but also chairs, beds, "Humongous" plushies, cabinets, certain rugs, certain tables (like the Gothic Tables), and some outdoor decorations.

Please let me know if anything I said was confusing or if you have further questions!