where's the bhustighre talking about gurungs and bahuns?
dude, it might not be a taboo in your culture but scientifically it's unapproved of. inbreeding (marrying, sexing and making babies with a close relative) produces messy babies - because both dad and mom have very similar genetic makeup. this means if there are any disease prone genes (often recessive) in your family pool, they'll be expressed and widespread in all your babies and their babies' pedigrees.
that's why most cultures in the world avoid and discourage inbreeding. there are reasons behind most of our social laws and practices. reasoning should not be limited to bahun/chhetri or hindu/muslim confinaries.