This is why the gov't should have no say in marraige.
If gays are alloweed to marry, I see no reason that people cannot have an incest relationship. Or marry more than one person.
Otherwise, keep it as it has always been.
Personally, I'd just like to see them say 'If you can find a religious place to marry (or whatever the equivalent is in that religion), we'll consider you two (or three or four or more) to be a couple.'