Actually I have said all along that we should look at the various things that do disqualify people from being married and determine we don't allow them. As long as they still ensure the main point of why the government would want to encourage/be involved in marriage at all, which is stability, then I don't really have much of an issue with them.
I have said that the current marriage will not work for polygamists. It just can't. But with some minor adjustments, we could make a separate set of marriage laws for those seeking multiple marriages. It would be the responsibility of those who are seeking polygamy to argue their side and provide reasonable solutions to the problems that people bring up concerning their marriages.
And it is not needed, for the most part, and has a couple of issues that need to be addressed if it is given to blood relatives, such as siblings or children/parent relationships, whether the relationship is sexual or not. The issues would be different for whether the relationship is sexual or strictly platonic. If it is sexual, then you are increasing the risk of certain learning disorders and genetic defects, but more importantly, it must be asked when the relationship started to form. Many of such relationships formed prior to at least one of the two participants being of consenting age and involves authoritative influence. If the relationship is simply for convenience, then you reach the issue that the marriage is really not doing a lot to promote stability, since it is likely that the couple will not stay together their lifetime. You also have the issue that it is wrong to invade the privacy of the couple but there is no way to ensure otherwise that the relationship is strictly platonic. A better arrangement for platonic family relationships that want more protection would be to offer some other contract. I would not advocate calling it marriage since the word "marriage" does imply intimacy, but wouldn't really care one way or another. I really don't see a big outcry for siblings or parent/child marriages being legal, so I say we wait til we have such a group who is able to reasonably argue their side and what they want.
The gay marriage advocates have already given their side, including arguments and implementation that is completely reasonable in how to implement SSM. It is up to the advocates of those other groups to present their own arguments. No one says that SSM should come just because it is the fair thing to do. There are plenty of reasons to allow SSM. There are no reasonable arguments, when it is compared to the exact way that OSM is currently available, to deny SSM.