Let's consider the implications for a moment.
The reason they didn't already have a dog, is because they lived in an apartment that doesn't allow pets.
As you can see from the furniture placement, they've moved. Perhaps into a proper house even!
And since they've had Kurosuke "since he was a puppy", this wording implying they've had him for quite some time, it happened years ago.
So, the timeline is- [Present Day]-->[Moving House]-->[Man's Best Friend]-->[Ponytail Over Shoulder Takagi]
To wrap things up, they've got a house with a white picket fence, a dog, and one child. Now they need 1.5 more and they'll have achieved the American Dream.
So get to babymaking, Nishitaka! There's still some time left!