[quote=@Penny]
This again assumed that the purpose of marriage is genetic. 
[/quote]
I'm talking about the future of the species if society deems it ok, and any kind of coupling that bring offspring, not just marriage. 

[quote]If we teleported into Cathamber's magic wonderland of easy genetic engineering would the moral objection still exist? What if they were sterile? Is there any grounds to object to it other than social taboo because that same taboo applied (and applies) to same sex marriage.[/quote]
It would still be icky in society, and we as a species would naturally be disinclined to it because of our evolutionary history and instincts. However, if the genetics were modified for their children, there was no religion against it, nor cultural taboos, then if I had to guess there would be a small population of genetically odd individuals that don't have the same genetic coding as most do, so they find their sister fuckable!