Wow, I have to say, this was not the reaction or advice I expected! In some ways though it's perfect, because I was not thinking along these lines at all! Thank you all for your advice and well-wishes. I'd like to address a few points here.
...saving some $ in exchange for alienating a partner and being legally connected to blue sounds like trouble.
Economics is in fact the main reason I see for anyone to get legally married. What other possible reason would there be for a legal marriage (as opposed to a ceremonial marriage)? The thing is, putting it like "saving some $" trivializes the situation. The monetary benefits of being married could add up dramatically over the years, and it's not like we are greedy rich people here. We are NOT well off and just trying to make it. Yes we could get bottom of the barrel health insurance for Blue through Obamacare, but we cannot afford the cost of insurance that would actually prevent Blue from going deeply into medical debt in the event of an emergency.
My employer does not recognize domestic partnerships. Really, marriage is the only way we see to provide good insurance for Blue.
...some of what you've said here could, I imagine, make Pink uncomfortable about this...
There is nothing here that she and I haven't discussed already, and I will probably show her this thread when I see her next. Yes, Blue and I would probably gain significant couple privilege if we were married. However, Pink and I can never have a lot of the benefits that Blue and I could have. Pink's family will never, ever accept me as her partner. It simply will never happen. Even if we got legally married in the future, in many states/countries, we still wouldn't be considered as married, including in this state that we live in currently and that all our families live in. I cannot change the world or even her family. Pink's family is a very important part of her life and she really cares what they think.
If the ceremony is important but you can live without the legal document, that's definitely an option that would let you "marry" both partners.
It's not about the ceremony. Neither Blue not I give a shit about that. Pink may in the future but we are not at a point where we are actually talking about getting married! That's key to this problem - marriage right now between me and Pink, legal or ceremonial, is not actually on the table in the here and now.
What if you marry Blue, but someday Pink ends up without insurance through a job? What would you do then?
This is a hypothetical. In the real world, Blue needs insurance very soon. Pink may or may not even be in this situation, ever. And in the future, I can expect I will be better off than I am now (I am just finishing school...) and more able to help those I love.
It's worth pointing out that Pink is actively seeking another long-term, life-entangled partner, potentially for marriage, and being married to someone is a life goal for her. I totally support her in this. So I would hope that if this future situation comes up, she will have a spouse then to help her.
It's also a little tough for me because I know there's something that Pink really wants that I can never, ever give her - which is a marriage that her family approves of. Simply impossible. Honestly, it makes me feel a little insecure about her ever deciding she wants to marry me... and this is something I just realized... hopefully her and I talking about that will help the situation is some way.
...it's understandable that she's concerned.
Absolutely. All the stuff you and others pointed out is very valid. The bottom line, though, is that I can only marry one person. It's a little weird to me that the consensus thus far seems to be that poly people just shouldn't get married? Yet, in another thread on this forum, I saw a poll and it looks like the significant majority of this forum is in fact themselves married! I'd love to hear from some married poly people about how that works out for them...