If the government is going to be in the business of legally recognizing marriages, it needs to do so consistently. If we are going to break with traditional notions of marriage as between a man and a woman (and I don't see why we can't) then it would be hypocritical to keep other arbitrary restrictions. The legal complications are certainly surmountable, as long as the necessary conditions are set (all partners have to consent, things like inheritance and alimony have to be determined ahead of time, etc.)
As for the morality of it - well a) I don't think anyone here wants my views on which relationships are immoral, to be the basis for government recognition (I don't even claim this) and b) I have long felt that a union can be called a "marriage" whether or not the government recognizes it. If 3 guys want to marry each other, no one can stop them - whether they get marriage licenses for it, is a different story altogether, but why shouldn't they?
I would even go so far as to say that the government might as well recognize incestuous unions (at least between siblings; between parents and children, there are serious issues of consent that get raised even if the child is an adult). In fact, my views of incestuous and polygamous unions are pretty much the same as my views on same-sex marriage.