Yes, there's nothing inherently wrong with the idea of a polyamorous marriage.
No, not at all, and if it ever comes to it then I don't think I would support the concept/institution of marriage.
You realize that this is the exact same thing conservatives said about gay marriage when it began to gain traction, correct? (ie that we should abolish marriage instead of legalize gay marriage)
Except that SSM and polygamy are two very different things and two very different institutions. The former is the extension of a right, the latter is the collection of human beings for a harem.
Now, if a person wants to do a threesome, foursome, fivesome, I don't care, but it should not be given legal status.