Hello! I've been searching for a few hours now and I'm sort of on the verge of giving up because Google blushes and refuses to admit that this fic exists, and I can't seem to find the old post on this comm that led me to the fic to begin with (some years ago). I'm really hoping someone is either a super-sleuth or better with their bookmarks than I am!
Essentially, as far as I can remember, this was a fabulous, fairly short fic series in which Reid, instead of submitting some sort of assignment to Hotch, accidentally gave him a DVD of his and Morgan's home-made sex tape, or something like that. Hotch is super intrigued, develops this kind of stalker-y crush on Reid, but it's all quite sweet, except where he's jerkin' it to the video like it's his last night with his right hand. Anyway, eventually he's caught - da-dum! - and Reid + Morgan sit him down for a nice chat about boundaries. This, according to standard slashfic logic, naturally evolves into a threesome, or perhaps just exhibitionism, whatever, you hopefully get the gist.
Pretty sure it was written by a big name in the fandom, but I've checked quite a few livejournals to no avail....Alas! Earwax.
Thanks guys, you keep on rocking! And happy thanksgiving to you fabulous people, you know who you are!
Edit: please see link in comments!