Adapting the law on man-woman unions to also cover man-man unions does not involve any technical hurdles on the legal side, it's just applying existing laws in a slightly different context.
Marriage with more than two people, however, would require many changes to the text of the law (except in places where the law already covers polygamy).
The burden of changing many legal texts is not nearly a sufficient reason to deny some people equal legal rights.
A thought occurred to me today as I skimmed an article in a rationality forum where the subject of gay marriage cropped up; seeing as the issue has been hotly contested in various public fora and especially the courts, what about poly? After all, many if not all the arguments for gay marriage apply to poly marriage as well.
Questions for LWers who are currently in a such a relationship, or have an opinion to share:
Do polies want to marry each other or do such relationships not lend themselves to permanence above a threshold of partners? Should polies campaign for the right for a civil union anyway? what are the up and down sides of this? etc