I can absolutely understand the feeling of being now a different status, a third wheel while they're being all official. It might feel to you like he's not treating the two of you as equals.
But you say you're not a marriage kind of girl. Surely your boyfriend knows that? Isn't it possible that specifically because you wouldn't want to marry him anyways, he feels it's not harming you or taking an opportunity from you? You can't marry him anymore, but you had no interest in doing so.
It's true that being married means they'll have a different status according to law. But you won't have a lesser one. All the advantages you're not getting, you already didn't have. Do you feel like it would be more fair if he was marrying you both? But you don't want to get married, right? Is it the fact that he "chose" her that hurts? But it's not a choice if she wants it and you don't.
I think it's important to talk with them about everything. Tell them you appreciate how they're trying to hide things like with the calendar, but you want to stay honest between the three of you, as you already feel left out right now an secrets you're not a part of would make things worse.
Then if feeling left out is your problem, ask how you can feel more like part of it. Be a first maid. Are you openly poly? If so, that could be part of the vows and you could be mentioned. Would you want to also wear a ring for symbolic reasons despite the lack of ceremony? Do you want a nonofficial ceremony such as handfasting?
Maybe talking about all of that will have you understand what is making you feel this way. It does make a lot of sense, they're making their bond stronger and yours is staying the same, so it seems weaker in comparison. Then work at strengthening the bond between you and him, and also between you and her. Tell her you appreciate how nice she's being, but sometimes you feel that it's because you're not a threat anymore and that makes you negligible, and so being nice hurts a bit when you think about the reasons for it.
Maybe talking about that with her will have her explain how she feels better, or understand you better.