Not sure if this belongs here or not...
I was doing a search on Yahoogroups and came across this gem of a group description.

The group is here

The description:
"A place for lovers of slash in Harry Potter. Pairings are limited, because there are just some pairings I will NOT have posted onto this site, okay? Example
- any Voldemort/? fics. Or Dumbledore/? fics. 'Cause that's just gross, okay? Main pairings will be - Harry/Draco, Harry/Draco/Ron, Harry/Ron, Ron/Draco,
Percy/Oliver, Harry/Snape, Remus/Sirius, Remus/Snape, Remus/Sirius/Snape, Fred/George, Harry/Sirius, Harry/Remus/Snape, Harry/Remus/Sirius, Ginny/Hermione.
Feel free to post your fics/pics, and comments about anything on the site. But, please, no nasty stuff, okay? Example - non-con. Oh, and put in the header
of a fic the rating and pairings and title of the fic, okay? Just so we all know the basics. Well, wind to thy wings! Esca *^_-*"

So twincest is OK, but sex with old people is not...Could someone explain the logic here?