No it wasn't. It was only 'hinted at' starting this season. Lecherous boys (for the most part) and women wanting representation kept demanding it on the internet and they caved. Don't try and change history to make it sound like "it was always planned".
That said, I have no problem with it even though I would have preferred it if Mako manned up and patched his romantic relationship with Korra.
While it is doubtful that I will change your mind you at least deserve an answer.
I don't care about sex at all. For me it made sense that Korra and Asami became a couple because of the emotional components. They just mesh well.
Starting with season two, Asami is the CEO of a large company. She probably owns half of Republic City. Despite all her responsibilities and demands on her time she -always- makes time for Korra, never says anything about being busy and never mentions that every day away from work is costing her money. How many CEOs would do that for "just a friend"? And go as far as risking their life when they aren't even involved directly?
Look at their interactions. When Asami talks to Team Avatar, she always looks at Korra. Even when she's together with Mako (again).
Or how about the fact that the stunningly beautiful, highly intelligent, refined and immensely rich Asami was single the entire time she wasn't with Mako? She waited 2 years for Korra to come back. If they had just been friends, then Asami could have dated someone else without a problem. Asami even offered to go with Korra to the south pole and care for her, leaving everything else behind.
Asami was the only one Korra confided in when she finally answered the hundreds of letters she received. Mako and Bolin are very good friends of her, but Asami is more than that.
It may not have been planned right from the start but when you write characters, they often develop their own voice. What may have been intented as a deep friendship at first becomes more in context because the character's evolution leads them to that conclusion. If the writers go against that, then the character feels wrong somehow.
Mako did man up and he confessed his feelings to Korra. Did you notice her face then? It clearly wasn't the "Oh yes, we will always be together!" reaction that would have lead to them patching up their relationship as a couple. It is obvious they both have feelings for another but they know they wouldn't work out as a couple and have known that since the end of season 2.
Unless you believe that Korra should have fallen for him again because of his earnest feelings I see no reason to believe that Mako & Korra could have happened.
It's sad for Mako, yes. But is it that bad for them to have a deep friendship and unwavering loyalty for each other? Honestly, this was better for them because as a couple they just don't work.
There was a lot of build up for Korra and Asami and while most of it happened in the last season, there were strong hints for it throughout the show.
I admit it is difficult to determine if it was friendship or more in the early beginnings because we'd need to hear their thoughts to know for sure. But romantic feelings developing out of a deep friendship? That's beautiful. I would say the same if it was Bolin or Mako btw.