People are bad at predicting what legal protections they may need in the future, and marriage covers a lot of them. Boilerplates are useful even in fields where there is pretty arbitrary freedom to customize. I didn't try to write my own contract with the Cryonics Institute, for instance, though as far as I know they and I are entitled to enter into any contractual obligation we'd like.
Marriage isn't just boilerplate for private agreements. It gives married people legal privileges: custody of children, hospital visiting rights, rights to decide partner's medical treatment in an emergency, tax reductions in some jurisdictions (usually for joint-owned property or for inheritance between the married partners), and so on.
Lack of good boilerplate for marriage-like agreements isn't the problem. In many states, private agreements cannot grant most of these legal rights. That's why abolishing government-defined and -controlled marriage is a prerequisite to equal-rights private agreements.
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