Homosexual incest is the probable door through which a general right to incest is discovered. What is the compelling state interest in prohibiting sodomy between two adult brothers, or sisters?
As of Thursday, there is none. From there, we're an equal protection argument away from a general right to consensual adult incest.
The genetic objection to a legalization of incest is already on shaky ground, since most incest doesn't result in birth defects. Those who want to discover a right to sodomy, but hold the ground on incest because of the possibililty of birth defects, will have to cartwheel like a gymnast to explain why there isn't a state interest in regulating all extramarital heterosexual sex, since STDs can also induce birth defects.