it is never impossible i went out with my childhood sweetheart/best friend, but i know what you mean, i have a few friends now who are definately in the friend zone.
my ex infact thought me and him were in the friend zone....personally i had always belived it was a zone constructed by men to avoid meaningful relationships with people they actually care about.
But as a veiwpoint, jumping into bed when drunk isnt wise, and further to that with a friend it can never feel right (dispite monica and chandler). Its not a case that you couldnt have had great sex its that you were drunk, and thus couldnt have possible been completely aware that you were sleeping with your best friend and couldnt have thought it through,
I feel its not a case that there is never a time when there is the friendship zone, but pushing the boundaries of any friendship is not a wise move when drunk.