I remember a little of this fic but I hope that would be enough to you wonderful people to know what I want. I think Dean got cursed by ??? not sure. To save him, Dean had to fuck somebody and be fucked. Sam, Dean and a girl had sex together but was not enough because there was something about "family secret desire". I know Sam wished that Dean was on top of him and Dean wished John was on top. And John too, he wanted to have sex with them (or only Dean) but didn`t dare.
Oh God, did somebody understood what story is it?