"Jim, gay marriage was considered an affront to public decency until recently."
As was the natural fact of being gay - in fact, up to comparatively recently it was a punishable criminal offence and there are those who wish that was still the case - just as there are those who would with to return to capital punishment, dog-fighting and bear-baiting.
The present campaign seeks to close an anomaly in the law and, not surprisingly, much of the opposition is generated by celibate mystics who would retain the right they have had to instruct us on how we should behave in our own beds.   
"Why is it an affront to public decency if siblings seek to marry?"
Incest is historically one of the great taboos of society for all sorts of reasons, cultural and practical.
I have to say, I find this discussion more and more revealing the longer it goes on - pity we're going to have to nip it in the bud for fear of closing yet another thread- unless that is your wish, of course!!
Jim Carroll