It wasn't that he was jealous, exactly. Not even certain he'd be upset if Luffy asked one of the others to join in somehow. For instance, the captain was always hugging Usopp, the two of them tumbling around the deck like laughing puppies. That wasn't something Zoro could provide. What if Luffy wanted similar stuff in bed?
Or the girls. Even he'd made big eyes at the Pirate Empress, although he hurried to put on an indifferent face before the perverted cook noticed. And nobody could argue that Luffy cared a lot for Nami, who was soft and flirty (when she wasn't being a greedy witch). All those things that Zoro knew he wasn't.
Luffy never said anything even hinting that he wanted such intimacy with other crew members. The problem was, he'd never really said anything about wanting intimacy with Zoro either. They just happened. A playful bit of 'sorry I thought you randomly attacked our hosts' jostling after Whiskey Peak turned into snuggling together against desert cold on their way to Alabasta.
In the palace the rest of them compared notes on what had actually happened while Luffy slept off his injuries. And suddenly Zoro had to fight back a major freakout (He put a fucking HOOK through Luffy!?) and one thing led to another. A few nights later, he found himself wrestling his captain into bed. Just to make absolutely sure that he wasn't suffering from any nasty side-effects from the poisoning.
They didn't do much talking that night. Or later. But Luffy'd laughed and enthusiastically licked Zoro in all the right places, so he figured he didn't really mind. Anyway, when it came to hand-to-hand wrestling the captain could kick his ass all the way to Raftel if he wanted to, and they where both perfectly aware of that.
Somehow, they continued to happen. Pre-fight warmup sex, post-fight victory sex, post-really-bloody-brutal-fights are-you-still-alive(-am-I?) sex and I'm-so-bored-let's-have-sex sex. Luffy learned to watch his teeth while giving blow jobs, Zoro learned that rubber-powered body on body bondage was the kink he never knew he had and how had he ever survive without thist knowledge? Meanwhile, everyone else learned to knock. Loudly. After that time they ran out of butter, cooking oil, Chopper's medical lubrication and Usopp's grade-A paint thinning oil, even Nami relented and added lube to the ship's budget.
Not once did Luffy actually say something. Nothing about emotions or promises or time, beyond the stuff he said to all of them. To be fair, neither did Zoro.
But – and this was a big but, which he unfortunately came to realize a few days after they left the Florian Triangle, when it was a bit too late to change things. Because, as Zoro discovered, the problem with making a big self-sacrifice for the one you love is that, if you first fail to die and then refuse to talk about your actions with the person you sacrificed yourself for? Then your actions are pretty useless both seen as love confession and you've also dealt a really hard blow to your internal mantra of 'nah we're not in love, heh, it's all just good sex'.
So. There was that little mess.
Before he figured out what to do about it, that utter pisshead of a Warlord of the Sea came back IN TRIPLICATE and the damn admirals and probably the entire bloody Marine corps too, only Zoro got blasted away before those assholes had time to arrive. Suddenly he wasn't just in serious pain, but lonely, lost and stalked by a crazy ghost-girl.
Oh, and while he ran around fighting monkeys, the love of his life went through horrible battles to save the big brother he worshipped, only to fail at it, after which he had to be carried from the battlefield in a catatonic state. To which Zoro could only say 'fuck you very much, fate' and up his training to even more insane levels.
So when Luffy came to their reunion smiling and laughing and even sexier than in Zoro's memories (goddammit) he became utterly tongue-tied from sheer silly happiness.
The first time he had watch, Luffy turned up in the crows nest and proceeded to cheerfully climb all over him. So, he figured that talking might be a bit overrated. Then they spent something like six hours having reunion-sex in the Sea King's palace, during which Luffy taught him all about the uses of Observation Haki in bed and wouldn't stop giggling even when his mouth was full of cock and Zoro started to feel a lot more secure in himself and his position around the captain.
Only for Luffy to run off the very next day, to climb all over the Mermaid Princess and her unbloodybelievably huge and smooth boobs. After which he ran around and hugged the stuffing out of Chopper and play-wrestled with Usopp and had a number of foodgasms when Sanji showed off his new skills... Well, forgive a man for feeling a bit confused. Zoro hadn't been hired for his brains anyway.
A while later, they were having story-telling time, at Brook's insistence (he claimed to be working on a rock-opera called Mugiwara The Musical, whatever that meant). After at least three interruptions in the latest story, Robin was forced to arm-lock Sanji while Luffy finally finished his tale about the (hot) Amazon Queen who kept feeding him yummy stuff.
Zoro was absolutely not jealous, pfah, he didn't do that kind of crap. No, it was merely shock and surprise that made him choke on his sake when Luffy blithely told them about how the empress asked to marry him.
"She wants to marry you?" Nami's gasp was barely audible over Sanji's howl of outrage.
"You lucky bastard," Franky said, shaking his head with stunned disbelief. "You DOG, Strawhat! What's she like, huh, huh?"
"Hancock?" Luffy scratched his nose. "Oh, she's nice. Bit weird."
Zoro would only notice that he'd bit through his sake bottle when Chopper started yelling about bleeding mouths and doctors, but that was still several minutes into the future. Right now, he could only stare in dumb shock at the his captain.
In a way, Sanji's shrieking was actually peaceful. It kind of drowned out the ringing in his ears, if nothing else.
"So, you and the pirate empress Boa Hancock..." Nami looked over at Zoro and then turned back to Luffy. "Are you – I mean, did you say yes?"
Luffy frowned and tilted his head. "Eh? Why would I do that?"
"Wellll, she's an awfully good-looking woman. And you and, uh... I mean, it wasn't that serious between you two? Or?"
Looking back at the event, Zoro would have to guess that this was when he started chewing on the piece of glass in his mouth. At the time, he'd thought the crunching, crushing sounds and the taste of blood meant something entirely different.
"Hammock? Me, with Hancock?"
And Monkey D Luffy, the man who would become Pirate King, threw his head back and laughed until he fell over. He kept laughing, paroxysms of hilarity making him roll all over Usopp's plate. Robin's coffee was saved from a similar fate only through a pair of swiftly-grown hands.
"-hishishi, whooa, Nami, my stomach hurts now!" Luffy said, wiping away a tear. "That's the funniest joke I've heard in forever!!"
"So you didn't do anything?" Franky asked, sounding a bit disappointed.
Sanji appeared to have passed out. Whether from relief or burning jealousy was impossible to say.
"Nah, why should I?" When Luffy, flush with laughter, favored Zoro with a blinding grin, the entire crew turned as one to stare at the swordsman.
Robin smiled knowingly and Usopp gave him a nervous thumbs-up, while Zoro himself only stared back in numb confusion.
"I picked Zoro right at the beginning, didn't I? And he's still here, so why would I need anyone else?"
I decided. I want you.
At that moment, Chopper finally noticed the blood dripping down his chin and loudly freaked out. Though tiny shards of glass were still stuck between his teeth, Zoro couldn't hold back his own smile, not even when Chopper began yanking at his arm in acute panic.
Because Luffy was looking at him and there, in that smile – in eyes that always searched for him first when the going got tough, when the burden of leadership became heavy, when even the invincible Pirate King needed someone to hold... there were the words he'd been too stupid to hear all this time. Right where they'd been, from their very first day together.