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I asked a similar question recently, and it occurred to me that we only see Harry feeling attraction towards Ginny in Harry Potter and the Half-Blood Prince (emphases mine):

They chose the one nearest a gold-colored cauldron that was emitting one of the most seductive scents Harry had ever inhaled: Somehow it reminded him simultaneously of treacle tart, the woody smell of a broomstick handle, and something flowery he thought he might have smelled at the Burrow. He found that he was breathing very slowly and deeply and that the potion’s fumes seemed to be filling him up like drink. A great contentment stole over him; he grinned across at Ron, who grinned back lazily.

and a little while later

“Excellent, excellent! Now, this one here... yes, my dear?” said Slughorn, now looking slightly bemused, as Hermione’s hand punched the air again.

“It’s Amortentia!”

“It is indeed. It seems almost foolish to ask,” said Slughorn, who was looking mightily impressed, “but I assume you know what it does?”

“It’s the most powerful love potion in the world!” said Hermione.

“Quite right! You recognized it, I suppose, by its distinctive mother-of-pearl sheen?”

“And the steam rising in characteristic spirals,” said Hermione enthusiastically, “and it’s supposed to smell differently to each of according to what attracts us, and I can smell freshly mown grass and new parchment and -”

So we know that something with a flowery smell at the Burrow attracts Harry. These next excerpts show that it's not their garden:

“Hang on,” said a voice close by Harry’s left ear and he caught a sudden waft of that flowery smell he had picked up in Slughorn’s dungeon. He looked around and saw that Ginny had joined them. “Did I hear right? You’ve been taking orders from something someone wrote in a book, Harry?”

and finally

Only as he walked blindly back through the crowd did he realize, from a trace of flowery scent on the air, that it was Ginny who was leading him back into the castle. Incomprehensible voices battered him, sobs and shouts and wails stabbed the night, but Harry and Ginny walked on, back up the steps into the entrance hall.

So was it only after Harry Potter and the Order of the Phoenix that J.K. Rowling decided to put Harry and Ginny together or did she decide it before, but only got it into motion in Harry Potter and the Half-Blood Prince?

Note that Harry doesn't show any signs of attraction to Ginny in Harry Potter and the Order of the Phoenix.

This was planned years before Book 6

In an interview at Leaky Cauldron in 2005, Rowling revealed that she intended for these two character to end up together, because Harry needed someone "who can stand the demands of being with Harry Potter". They were just not emotionally mature enough from the beginning and that's why this relationship was in a way delayed.

MA: Had you been trying to get them —

JKR: Well I always knew that that was going to happen, that they were going to come together and then part.

ES: Were you always -----ing it? [We can’t figure out what Emerson actually said here.]

JKR: Well, no, not really, because the plan was, which I really hope I fulfilled, is that the reader, like Harry, would gradually discover Ginny as pretty much the ideal girl for Harry. She's tough, not in an unpleasant way, but she's gutsy. He needs to be with someone who can stand the demands of being with Harry Potter, because he's a scary boyfriend in a lot of ways. He's a marked man. I think she's funny, and I think that she's very warm and compassionate.

These are all things that Harry requires in his ideal woman. But, I felt — and I'm talking years ago when all this was planned — initially, she's terrified by his image. I mean, he's a bit of a rock god to her when she sees him first, at 10 or 11, and he's this famous boy. So Ginny had to go through a journey as well. And rather like with Ron, I didn’t want Ginny to be the first girl that Harry ever kissed. That's something I meant to say, and it's kind of tied in.

It was intentional for Ginny to show her affection for Harry to the readers much earlier (Harry Potter and the Chamber of Secrets) but really end up with him after going through maturing enough and becoming the "ideal woman" for him.

Moreover, Harry had to go through his own emotional journey as well, before being, simply put, "ready".

One of the ways in which I tried to show that Harry has done a lot of growing up — in “Phoenix,” remember when Cho comes into the compartment, and he thinks, ‘I wish I could have been discovered sitting with better people,’ basically? He's with Luna and Neville. So literally the identical thing happens in “Prince,” and he's with Luna and Neville again, but this time, he has grown up, and as far as he's concerned he is with two of the coolest people on the train. They may not look that cool. Harry has really grown. And I feel that Ginny and Harry, in this book, they are total equals. They are worthy of each other. They've both gone through a big emotional journey, and they've really got over a lot of delusions, to use your word, together.

_{"The Leaky Cauldron and Mugglenet interview Joanne Kathleen Rowling: Part Three," The Leaky Cauldron, 16 July 2005 [Emphasis mine]}