Would you support the marriage of a brother and sister if they were never going to have kids.

So i have been thinking a bit about who can get married etc with all this gay marriage debating going on. Then it occurred to me that you can't marry your relatives. I figure this is mostly to try reduce the amount of genetic disorders that get expressed but if you were never going to have kids, why not marry your sister or brother?

[EDIT] Reddit on average supports consensual incest.