Do you support a brother/sister marriage? Polygamous marriages?
Well, at least your asking me this time. Rather than telling me what my opinion is.
I don't really have an opinion on brother and sister marriage. I don't know enough about the topic to discuss it intelligently.
Our laws aren't set up for poly marriage, so we have no way of regulating it. If two people are married and they add a third, are both married to the third person, or only one of them? If one of the three wants a divorce, does that mean that the other two are no longer married once the divorce goes through? Or the other two are still married? If you divorce, does property get split 3 ways? Or does it get split based on the time in the relationship? Does every member of a poly marriage have conjugal rights to every other member? What if one person wanted to add a third, but the other didn't?
We have no precedent for any of this. Nor any laws to cover it. And the issues get more wildly complicated the larger the number of participants grow.
We have answers for every such question in two person marriage. And all the same rules apply for gays as apply for straights.
There need be no adjustment to any law save that you stop excluding gays.
Poly marriage would require massive changes to our laws, and all new precedent for hundreds of unresolved legal issues that our law has no tools to resolve.