Both. Descriptively, the practical difficulty of changing the law isn't the reason the law hasn't been changed in this way; the reason is that a great many people would oppose it on religious, moral, and other normative grounds. Many other laws have been changed over time despite the changes being nontrivial to implement, because the necessary people agreed in those cases that changing the laws was for the best.
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