It is only possible to choose so when you're buying, I'm afraid. They added the ability to preview homes in 'unfurnished' as well as 'furnished' versions, so people can see and decide based on that. But there is no way to change your mind after you make the purchase.
However, almost all items that come with 'furnished' houses can be crafted and bought from other players. So you can get your house furnished the same way, it will just take longer to hunt down the items and place them than if you had bought it furnished, but it's usually much cheaper to furnish it yourself. If you lack the gold for it, you could take the Crown difference that the furnished house would have cost you, and exchange them for gold via a Crown Store gifting exchange. You usually end up with more bang for your buck that way.
The furnishings that come with a house purchased for Crowns are also Bound, so furnishing a house yourself with crafted and gold-bought furniture gives you more control, and allows you to sell the items if you ever re-decorate. Of course, you can also buy the furniture directly from the Editor menu for Crowns, it will just cost more to buy them individually than the Crown difference that they charge for the 'furnished' version. But this way you only get the items you'd like in your house, and don't end up with lots of bound furniture you may not even want.
So, sorry to disappoint you, but I hope you'll enjoy decorating your house regardless!