At first, it was just a way for officers to blow off steam.
Well, more accurately, it started with an energetic discussion about old Earth movies and tv shows between Kirk and Uhura, which lead to a discussion about pairings, which lead to a discussion about slash pairings, which lead to Kirk creating an Enterprise-wide fan-fiction server, which lead to a way for officers to blow off steam.
Spock was doing eyebrow number 7,"What the fucking fuck is this fuckery?" when he found out about it a while later ("Who is 'TJeff' and why is he...sleeping with John Adams?"), but became resigned to it after a while, pending the occasional question ("What is a McShepp?").
And then? The RPF started.
Bones bore it with his usual grace, stabbing Kirk with the occasional hypospray of vaccinations "You're due for, Jim, now stop whining." Spock continued to raise his eyebrow, especially since pairings of Spock/Randomcrewmember and Kirk/Randomcrewmember were neck in neck in popularity.
The day It happened, however, just happened to be the day Kirk was hiding from Yeoman Rand in the botany lab (Where was all this paperwork coming from, anyway?) and flicking through all the RPFs, giggling at things like McCoy/Spock.
He scrolled past it the first time, stopped, and painstakingly scrolled back up to the one with the pairing Kirk/Spock, written by Anonymous. He clicked the link.
10 minutes later, after reading it twice and staring at the screen blankly, Kirk started tapping again, this time going into the coding and finding the originating Padd. When he found it, he stopped. And stared.
Spock blinked up from the 3D chess set when his Padd beeped. He picked it up, tapping the screen to open up the email from Jim.
I liked your story. Meet at your place at 2100 hours to discuss it?
I believe that would be benificial for all parties involved.
Great. See yah then.
What broke out on Spock's face? Definitely not a smirk.