Luke swore to himself again and again that he didn't have a problem with it. Han and Leia falling in love was a wonderful thing, and he was happy for them-it just took some getting used to. Or maybe a lot of getting used to. First, after Bespin there was the realization that Leia hadn't lost a friend to the carbon freeze, but a lover. That was a shock, but he'd come to accept it in the months before Endor. But before Han's rescue, his and Leia's relationship was more an intellectual concept than a stark reality.
He'd realized that as soon as he and Leia landed on the tiny skiff above the dunes of Tattoine. As soon as Leia's feet hit the deck she'd run to Han and thrown her arms around him. Han's expression before he lowered his face to kiss her was like nothing Luke had ever seen before in him. Not a touch of sardonic wit, not a hint of sarcasm: just the look of a man who was very in love with a woman he hadn't seen in a very long time, delighted to feel her leaping into his arms. As happy as Luke was in this moment of triumph, when he looked around and saw the knowing smiles on Lando's and Chewie's faces, he thought, isn't this weird to anyone else?
In the days since then, Leia and Han's status as a couple started to feel normal, but there were times when Luke felt left behind, out in the cold while Han and Leia basked in their reunited bliss. He didn't begrudge them, really. But at those times their love felt abrasive, like a constant irritant, and not just in the realm of the senses, but in the Force as well.
The first time he felt something unique from Han and Leia in the Force was the day he watched them walk across the Home One hangar arm in arm. He and Han had been shooting the breeze, arguing about the best levels for booster optimization in atmo, when Leia came over to ask Luke about a reconnaissance mission. Business being over, his two friends (or rather his friend and his sister) headed off to "get a bite to eat." He'd felt enough of that vibe to refrain from mentioning that he was hungry, too.
Then they walked away, Han bent over to bring his head close to Leia's, her arm reaching around his waist. They were turned toward each other, completely in their own orbit. As strange as it felt, it looked utterly natural; the impression he got through the force was of an integrated unit, perfectly spherical and pulsing with a quiet but undeniable energy.
Something that right shouldn't be making him uncomfortable, but it did just the same. Luke would have defended himself up and down to anyone in the system that he wasn't jealous of Han for "getting the girl." In fact, he was just as jealous of Leia as he was of Han, and he was jealous because his two friends shared something that shut him out completely, leaving him on the outside looking in. But then, it didn't. It didn't shut him out completely. And that turned out to be the biggest problem of all. Because he'd begun to feel a burning curiosity: he'd never sensed something like this in his experience of the Force. He'd felt an energy between couples, but not like this. In an effort to understand, he found himself gently probing their bond.
He'd been so damned curious, hadn't he? So frustrated with his own ignorance, like a kid (like himself) taking apart a droid to see what made it run. But it was the role of a Jedi to understand all aspects of life, wasn't it? So he'd begun to consciously attune himself to that energy. His connection with Leia grew every day, yet neither of them really had any clue as to how it worked or how to approach it. Didn't he owe it to all of them to explore its manifestations? But having been established, Luke's attention was harder to control than he'd anticipated. The emotional link between him and Leia became nearly impossible to close.
At first, there were just vague sensations, things that were only the barest step beyond what careful observation could tell him. For instance, the sense of how their faces would meet when they kissed. He knew now which way Han and Leia each tilted their heads when they kissed, and then almost without trying, he knew which of them tended to open their lips first (Leia). He watched them hold hands, he knew what those hands looked like clasped together, but then suddenly he also knew who took the other's hand more often (Han), and who held on the tightest (Han again). Since Luke felt that these impressions were offering insight into how humans loved, he figured it was okay, especially since it was such little things, really-he didn't need to worry about it.
But then things began to escalate. One day during a discussion outside a briefing room of H1, the conversation turned into a contentious debate, and Han moved closer to Leia's side and put his hand on her back, a gesture that was becoming familiar to Luke. Leia didn't acknowledge it other than to step a hair closer to Han. In an instant their individual presences in the Force transformed into the glowing sphere of their combined energies.
When Luke saw Han's protective stance, saw Leia ease into it and accept his bolstering presence, an impression, a series of images and feelings and scents burst into Luke's head. Last night they had lain on their sides, Han curled around Leia from behind like a seashell (Luke had seen a holo once in school). He surrounded her, pushing into her at a steady andante pace at one end of the curve, and burying his face in her hair at the other. He murmured something in Leia's ear and she answered with a nod and a little hum. Her head rested on his bicep, and she held his forearm curled to her lips, where she nearly suckled his warm skin. Han's other hand was buried between her thighs, and what he was doing with his fingers there was as clear to Luke as the shape of their bodies curled together, or the colors of their hair mingled on the pillow. He perceived all this in the time it took to draw one breath.
Luke actually staggered where he stood, drawing looks from the company around him, including Leia and Han, peering out at him from their sphere, the one they thought only had room for them.
You might think that that experience taught Luke a lesson, showed him that it was time to back off, strengthen the walls around his consciousness. Probably he should also have spoken to Leia, diplomatically indicated that her sex life was becoming a bit of a problem for him, and could they work on increasing the mental boundary between them. But Luke was beginning to realize just what a stubborn son of a bantha he was, and what's more he was even more curious than before. He wanted to know how it all worked-not just Han and Leia's sexual bond, but the awareness of it that was growing in him. And maybe, if he was being really honest, that one taste of pleasure, of connection, wasn't enough. So although he didn't exactly pursue more encounters like that, he didn't exactly avoid them either.
And that's when things really started to get out of hand.