It wasn't lying, not really. It's just that whenever they started to get too close to the truth, Harry would bring up some totally unrelated but fascinatingly interesting fact to distract them, and they'd forget for another few weeks to ask about his strange behaviour. Or, to be more specific, his lack of apparent animosity towards his oldest enemy, Draco Malfoy.
After all, they still had no reason at all to trust or even like him. He still called Hermione a Mudblood whenever she came within hissing distance, still made fun of Ron's family and their woeful lack of fortune. Crabbe and Goyle still bullied for him, and he certainly didn't start being nice. But he did stop hurling insults at Harry, and picking fights, and every once in awhile Harry even caught Draco staring at him as though he were not, in fact, the worst thing in the world to be looking at.
Which was dangerous, and Harry actively tried to discourage. He'd hunt Draco down after a Potions during which his neck had itched constantly from the weight of Malfoy's eyes. Then he'd tuck them both into some dusty niche and cast a number of subtle spells to discourage one's attention from their little corner, and proceed to torment Draco to the edge over and over, then, often as not, leave him two strokes from orgasm and walk away.
After all, it certainly was an effective reminder, and neither of them could afford to let people think they liked each other. Not with what was on the line -- Draco's neck, now that his father had escaped from Azkaban, and Harry's two best friends, at the very least. Snape's cover might be lost, as well, for it was Snape who had spent the time and energy to show Draco that purity and status weren't everything, and cruelty and fear made for a pretty shoddy way of life, all things considered.
Harry had brought home those lessons with lips and tongue, teeth and hands, cock and ass, taking all of Draco's anger and fear, all his pride and hate and pain and giving it back as pleasure. They never spoke of anything so mundane as feelings, but Harry knew it was more than just the sex. Enough, anyway, that Draco had gone to Dumbledore and officially declared his defection, and brought Crabbe and Goyle with him. Even though this thing between them had started in anger, genuine affection had dulled that sharp edge long ago, replacing it with something far more lasting.
It was Draco's eyes, the grey gone stormy with longing, that enabled Harry to drive Snape out of his head week after week in Occlumency lessons. Draco's voice, rough with passion, that Harry used to block Voldemort's destructive whispers from his inner ear. Draco's skin anchored him, the heart beating against Harry's chest and the lips on his own that kept Harry here when Voldemort tried to lure him away. It was the taste of Draco's come on his tongue that washed away the ashes of future guilt whenever Harry thought about becoming a murderer.
Harry knew he'd have to tell Ron and Hermione soon, but as he buried his face in Draco's hair and smelled the strange sweet spices of his lover, he knew he'd keep this to himself as long as possible. They both had the irrational fear that, exposed to direct scrutiny, this thing between them would crumble, leaving them both less than before. Draco, perhaps, more than Harry, whose friends would offer forgiveness where Draco's family would seek vengeance, but it was a risk neither was willing to take.
For now, they continued to direct curious eyes elsewhere, hiding the elephant between them with smoke and mirrors for as long as it could last. Until someone stood up and pointed out that the magician still had the ball in his hand, that Harry obviously was in love with the Malfoy boy, they would stick to furtive gropes in dark niches, hasty blowjobs in empty corridors and the occasional hot, desperate fuck in an abandoned classroom. Because during those secret moments, they were both real, and nothing else mattered.