Scripter, you already know the answer to this question so why do you bring it up? Obviously, there are increased risks of mental and physical disabilities in children produced from parents who are closely related by blood. Of course, nothing currently prevents closely related men and women from producing children, without anyone even knowing that the child's mother and father are related by blood.
In this discussion, I have been focusing on the "public policy" choices of the matter. In other words, I prefer to look for ways to accommodate critics of same-sex marriage so that an agreement can be reached that would permit legalization of gay marriage by a general consensus within our society.
By contrast, you apparently want to focus upon the IDEOLOGY behind public policy disputes. That is: focus on the most contentious aspects that bring out personal values or principles (often expressed in absolute, no-exceptions terms) that separate opponents in public policy debates---because as ideological arguments are made, there is dramatically less possibility of reaching consensus.
With respect to your larger question of "why limit marriage to only 2 persons?" (even if unrelated by blood), personally I have not thought about this hypothetical very much so I cannot offer you a fully-developed opinion. But given my first principles (previously stated), I might be persuaded that other types of marriages should be recognized--even if they involve 3 or more persons.
Early-on in this debate, someone posted a message asking why government should be involved at all in this question. That is actually an interesting question which might be explored.
Apparently my question wasn't very clear. That question was in reference to same-sex consenting adults related by blood. Since you previously said:
[you] had no problem limiting marriage to two persons not related by blood.So my question was why? Why would you want to prevent two same-sex blood relatives from marrying. They can't have children so there's no risk of mental and physical disabilities in children.
...I might be persuaded that other types of marriages should be recognized--even if they involve 3 or more persons.
A couple of times in regards to same-sex marriage you've mentioned the American family can do so in an amicable, civil manner. With what America is saying across the nation in regards to same-sex marriage, I would think such a proposition would go over as well as a lead balloon.
I think we should always consider the effect social changes will have on children, who are not guinea pigs for social change.