I actually had trouble finding this document; I eventually hunted it down through a blog run by a lesbian. You'd think this would be easier to find, neh? The list isn't perfect, as it's the result of a text-search against law databases, nothing more. (If I haven't mentioned this elsewhere: we need better methods for finding laws relevant to a given situation!)

Although I'll leave the discussion for elsewhere, but it seems that these things come down to a few categories:
a) benefits you could just get through individual contracts (though the lack of automation does make this a pain) -- this includes things like making medical decisions, inheritance, etc. (though you would currently be unable to get the tax benefits married couples receive when a spouse dies)
b) benefits that could have been designed to be issued to the one person of your choice rather than automatically on the basis of marriage (most benefit-providing entities, like insurance companies, seem to like this "one" number)
c) benefits and obligations stemming from a "corporation" view of the family, where loans and grants are given to a group of people and rates, amounts, and schedules are based on their combined income (filing taxes jointly would fall in this category, for example)