Those "assumptions" of authority and responsibility are precisely why Spouse and I got married. I have very few blood-relatives, none of whom I trust to make decisions for me if something happened, nor do I want Spouse to lose "rights" to something that is "mine" if we didn't do paper-work on it (such as inheritance of our house which is in my name only at this time). Also, we want to be able to share our insurance benefits, etc. and for that, you have to be a "legal relation". These are examples of "assumptions of responsibility" that are offered by secular marriage.
We do NOT, however, subscribe to the "assumption of authority" placed upon legal/secular marriage because we don't believe that marriage = ownership.
I do not understand why some folks get bent out of shape when other people, especially strangers, make "assumptions" about marriage and other personal/relationship things, because we all tend to use our own frame of reference when it comes to making sense of the world we live in. If someone makes a "wrong" assumption about why I'm married (for example, I get "why did you get married if you don't want kids?" once in a while), it simply DOES NOT THREATEN ME AT ALL. In fact, it feels ridiculous trying to explain it or justify it so those people can understand, because it suggests that their opinion has some sort of merit in my reality.
So, go ahead everybody - assume anything you want about me. I'm sitting on my butt judging you too. At the end of the shift, you go your way, I'll go mine, and I'll keep doing what I'm doing and so will you. (Not very conducive to the "poly mentality" of "ohhhh I just loooooove everyone! group hug!" I know, but there you have it. It's a wonder anyone wants to spend time with me or be my friend, isn't it? ).