As ridiculed as the slippery slope argument against same-sex marriage (polygamous/incestuous/bestial marriages will be next) has been by its proponents, there aren't many serious reasons to maintain that the definition of legal marriage will then, having expanded to include those of the same sex, stay put forever after. And so the institution will reclinate back to its own past, the only ubiquitous purpose it represents becoming legal (and by extension political among the upper crust) in nature as was often the case in antiquity.
Divorce rates show that contemporary marriage isn't about lifelong commitment. Open marriages show that it isn't even about serial long-term commitment. Same-sex marriage shows that it isn't about procreation. Be prepared, as Jeremy Irons suggested, to be shown that marriage isn't about romantic love, as a father marries his son to hand junior the family fortune free from government confiscation. Polygamy will show that marriage isn't about a special devotion to one other person, as marriages of convenience already illustrate.
Better contact the financial planner to see whether or not we should get married, whatever our situation and whoever we may be. Perhaps the next step after that will be to annul any differentiation in legal or taxation status marriage confers and be rid of the whole mess forever.