The first time Gokudera realizes he loves Yamamoto is one special day when he looks up one afternoon to find Yamamoto staring at him. Since Yamamoto lacks any sort of subtlety the gaze is vaguely stalkerish and has a "I'm thinking how to screw you" feel.
In retaliation for this extremely inappropriate gaze, Gokudera puts dynamite in Yamamoto's underwear drawer.
Yamamoto ends up blowing off the middle finger of his left hand (it would have been deadly, but Yamamoto realizes the danger at the last second). Shamal is a genius at medical science, and is able to reattach the finger with little nerve damage. There is still a ring of scar tissue that surrounds the base, and it almost looks like a ring. After Tsuna forces Gokudera to apologize, Yamamoto laughs and says that at least now everyone will know who he belongs to.
Tsuna thinks that he means the Vongola, and laughs uncomfortably and feels a guilty pit open up in his stomach at the realization of how much this is costing his friends.
Gokudera doesn't miss the pointed gaze, and can't help but notice how the raised ring of tissue and stitches looks vaguely like a wedding ring on the wrong finger. His heart blooms a little like a flower (FUCK! Why is his brain so gay sometimes), and for the next month he watches Yamamoto back. Like a hawk. Like a super-watchful Italian Hawk.
That's actually how they end up in bed the first time.
It isn't really on purpose, at least on Gokudera's end.
Yamamoto has realized that he can't stand for Gokudera's attention to waver. If it leaves his being, he thinks he might wither and die. He has transferred his chosen audience from the school baseball team's supporters to the Vongola and especially to Gokudera.
He is sixteen and doesn't yet realize what love is. So he sits and antagonizes Gokudera as much as is possible, befriends him and adores him and watches him with all of his being. One day he wakes up hard and in agony and when he finishes getting himself off he is almost surprised when it is to the thought of silver hair and green eyes.
The stupid thing is that he gets hard from all that a lot more often – his cock feels like a hairpin trigger. He's glad his finger was reattached so easily, because without it he wouldn't be able to pull "the stranger" so easily, imagining a ring laden grasp instead.
And one day Yamamoto is watching Gokudera watch Yamamoto and he can't quite help the physical reaction.
He thinks he should still have shame – they're partnered together (as always) on the lookout for a major drug deal between two families, but as Gokudera would say he doesn't even know what shame is.
His pants get tighter and tighter, and he shifts uncomfortably because IT IS uncomfortable, so fucking uncomfortable. His pants are too tight because he's had yet another growth spurt and they tent obviously, there's no hiding this one.
Who can look away? It's like a really bloody car crash.
He puts his hand behind his head and laughs while waiting for the ensuing explosion.
Gokudera, with the air of staring at that car wreck and with the same horrified but curious expression reaches over and begins running his hand over the top, pushing the zip down into Yamamoto's cock. When Yamamoto's breath catches, Gokudera pulls down the zip and roughly grabs it out it out, running his hand up and down, up and down in an expert motion.
Yamamoto pulls them together, pushes his tongue onto Gokudera's in a slobbery and entirely unromantic kiss and then leans over to let the smaller body fuck him.
They miss the drug deal (Reborn punishes them something awful) and find something else.
Most often it feels like something stupid and blind, like they missed when the good brains came in and had to settle for each others.
One day Gokudera realizes that theirs is not an empty fuck, that they do in fact mean a lot to each other. He realizes this when he nearly blasts Hibari (Hibari! Danger, danger!) when the ruler of the old disciplinary committee makes what looks like a pass at Yamamoto (It is not a pass and is instead solely an ass-kicking. Gokudera's skewed version of the world misinterprets this).
He vows to give Yamamoto something he can't forget, to brand his being so far that he can never leave Gokudera's gravity again.
But that's another story.