We aren't really talking about literal deformities here. Most of the time it's usually genetic diseases. An uncle of mine married his cousin, their child had chronic leukemia which was eventually fatal..
Aside from the fact that you can't really have viable offspring and the so called social 'stigma' towards incest, I can't really think of anything extremely horrific about consensual inbreeding. Love is blind, after all, as cliche as that sounds.
It's the nature of my circuitry.