i'm not sure what it's called or who wrote it but this is what i remember from a fic that was similar to your description:
they were camping and the girls woke up alone feeling drugged (i think), inuyasha and miroku were missing but their weapons were left.
the boys wake up naked and chained to a wall, inuyasha is human as i believe it's the new moon. they find out they're going to be used as sacrifices and they have to be virgins so that the leader and his followers need virgin males to sacrifice so they can live forever or something.
the girls come and attack and free them...
i don't remember the name or the author, sorry.