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.
Boilerplates are great and as you say they are available and in common usage even when there are arbitrary legal restrictions preventing all other possible arrangements. So there is no practical reason that we need to be prevented from making our own contracts.
Current marriage laws and traditional implementations thereof by the legal system are hostile to my interests for reasons including but not limited to being grossly sexist. It would take rather extreme extenuating circumstances for me to be willing to enter into such a contract. Mind you this government interference isn't nearly as offensive to me as the laws preventing me from purchasing and using drugs as I choose including such things as anabolic androgenic steroids and cognitive enhancing drugs but at least the latter legal restrictions are of the kind that can be ignored in principle.
An example of a legal contract that cannot be entered in to: One partner agrees to have children with the other on the condition that in the case of separation they will not be obliged to provide ongoing financial payments to their former partner. Not being able to make such an arrangement means that couples who would otherwise have - and expect to benefit from - children. (Of course for signalling reasons current situations are more likely to describe their aversion in purely emotion based language without admitting to real reasons. The effect remains.)
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