This bit has an illustration by obstinate_fate, which can be found at my LiveJournal!
For the first few minutes of their sudden and totally unexpected affair, Roy thought reason #3 must be Fullmetal’s highly-skilled tongue. It certainly got plenty of exercise, hurling invectives at him across his desk, the very desk he was now sprawled half-beneath. Edward’s use of that tongue in an urgent and extraordinarily hot attempt to suck Roy’s tonsils out was far more pleasant than hearing a disjointed report.
Then, as layers of clothing were ripped away with a snarl, Roy thought perhaps reason #3 might be the automail hand that swept along his skin, cool at first between the sudden heat of their bodies, but warming quickly. He boggled at the fine motor control as metal fingers plucked his nipples to just the pleasant side of pain, as though he were a guitar or maybe a big string bass, strung tight and made to sing.
A few minutes later a sharp clap! sounded somewhere between Roy’s legs, and he thought maybe reason #3 was Ed’s alchemical genius, most notably his ability to make lube out of the dregs of Roy’s abandoned coffee. Then there was a finger inside him, followed by another, teasing him open with confident skill, and Roy thought that really he didn’t care what reason #3 was as long as Ed fucked him, soon, oh dear God.
He had a lucid moment a little while later and thought this, this must be reason #3, a combination of reasons 1 and 2, which was almost cheating, but oh fuck he didn’t care about that when Ed, his crazy, underage, contortionist subordinate Ed, could fuck him into next week and suck him off at the same time. He must’ve said some of that out loud, because the blonde’s eyes flicked up to catch Roy’s from behind a spray of long blonde lashes, and he hmmmed around Roy’s cock.
The last conscious thought Roy had was that yes, this must be reason #3, and damn, if he could have performed the same feat when he was fifteen, well, Maes probably wouldn’t have married Gracia after all.