I don't know if any of you remember this, but a few years ago HBO made a couple of movies called If These Walls Could Talk - there were two, and I really recall the second one, and one story in particular. It was about 2 lesbians back in the 1960s who were in their 70s or so. They'd been living together for years, but back then, people might speculate about two same-sex people living together, but rarely were they out.
One day, one of the women collapses and has a stroke. When this happens, her partner cannot be in her hospital room as she's not family. She cannot make any decisions about her partner's care. Then, when her partner dies, not only is she alone in her grief because she's "just a friend," she is unable to plan the funeral for the woman she loves - her partner's family takes over. And the final insult is that their house was in her partner's name, so the family sells it right from under her.
She loses everything - her life partner, her home, her belongings. (And yes, I did bawl like a baby and there's a damned good reason Vanessa Redgrave won an Emmy for this performance)
I will grant you that things are different now. Now, you can get a Durable Power of Attorney that will allow you to designate anyone as having say over your medical care. You can buy a house or other items jointly. You can do estate planning to make sure that your partner will get what they need should you die first. But my point is this - our society is set up so that for the most part, married couples don't have to make the extra effort to get these rights - they're automatic.
When you walk into a hospital, you don't have to say anything but "That's my wife/husband," and unless some other arrangements were made, that's who will be making your decisions. Marriage laws differ by state, but there are still a number of states where everything is automatically co-owned.
Marriage as we currently know it doesn't need any more protection. Same-sex and multiple-partner relationships need to be legally recognized. I hope that I'm preaching to the choir here, but if there's anyone who doesn't get why this matters, feel free to repeat this or point them here.