Okay, yesterday I watched Sorry Bro (S4E16) of HIMYM.

Now, I am SURE that I read a fic once where what Lily "came to give to Marshall" was actually something sexual (I think it was a blow job?). And she gave it to Barney, so he did the same to Marshall. Possibly in a cupboard???? I had a look yesterday, but I can't find this fic and I have no idea who wrote it. Does anyone know of it?