"3) Is a hat rack a consenting adult in your neck of the woods?"

Not really, but I was hoping to take out some life insurance...

"2) Polygamy? Why not, can not think of a reason against it if all parties are consensual. Could prove awkward benefit and legal wise, maybe those problems could be legislated against to accomodate the scenario"

A fairly managable issue since it's handled in other cultures and was handled in the US until the Federal government forced Utah to change its marriage laws.

"1) Could have medical implications if the relationship too close, this could be a valid reason not to legalise."

So the consenting adult thing only works for those who can't reproduce, not those who shouldn't? And recessive gene related birth defects are an issue for all couples. Should we reconsider marriage when it's know couples will have problems even if they aren't related?

Amos, Amos, you're right. But who said you have more right to decide for society than Mrs. Grundy? Remember the issue here is not limited to same-sex marriage. It will necessarily redefine marriage in ways we might not have considered. For example, if the Federal DOMA is struck down by the courts, the Federal anti-polygamy will inevitably follow.