i am going with this definition of incest (dictionary.com):
in·cest
1. Sexual relations between persons who are so closely related that
their marriage is illegal or forbidden by custom.
2. The statutory crime of sexual relations with such a near relative.
(american heritage dictionary)
and the common use of the term to include sexual activity between close relatives, such as siblings.
to repeat my question: in such a case, say two adult male siblings, what is the non-biological problem with it?
"adult" removes the "pedophilia" orthogonality. "male" removes the "feminism" orthogonality.
maybe i should add "twin" to deny reference to natural power relationships, caused by difference in age.
--ravi