Tangent, but do you have a reason why not, assuming they don't have kids?
Well, my reason for being anti-incest is because of the whole having kids issue - that their chances of having a genetic disease is X times higher than non-incest. Something like that, I did it last semester in Molecular Biology/Genetics but I honestly don't remember the exact facts.
But kids aside - there might be those who abuse the system. As a crude example, person A wants to benefit from family welfare (which pays two INDIVIDUALS differently to a COUPLE - couple gets more) but he can't exactly "fake" a marraige with an outsider because if they divorce, he would have to deal with issues such as splitting their assets. So person A decides to marry his sister, B, instead, so they both can benefit from the family welfare payment system and they both trust eachother enough that when they "divorce" they won't run into issues.
Isn't this just sanctioning discrimination?
"yes we have equal unions, but we also have a special type of commitment that only the heterosexuals can get"
I think under the law, we should have one union (I am pro gay marriage - if there were a referendum I would vote yes), but I also think that churches...etc. should be able to do whatever the hell they want and hence, "marry" whom they please...etc.
Just like how the orthodox church only marries two people of a certain religion or something or rather, I can't remember, but it's discriminatory...etc.