@failures art,
There is no code going on, just a good knowledge on Sociology that you don´t have...
1 - You still did not answer if you admit that there is such a thing as Social communal Property, what we usually call Culture, with specificity´s from place to place, community to community, that originate behavioural distinctions between groups...
2 - Why not to extend such rights to all kinds of Social fringe groups ?
...and license Marriage inside family or with children and so on...
The honest answer is,
just because a
VAST MAJORITY finds it unacceptable !
So bottom line is that there is a huge amount of Hypocrisy going on here !!!