I'm worried my girlfriend will end up marrying her other partner

I've been in a LD polyamorous relationship for almost two years. My girlfriend has a boyfriend (who I'm friends with) who she’s been with for much longer. I have problems dealing with insecurity, overthinking and jealousy of their daily life together. Despite her not wanting formalized relationships nor children, I have a brooding feeling that her boyfriend will eventually propose to her and they will end up married. For me it's a declaration of a personal choice of the better partner, also unlocking many societal privileges. I simply don't want to date anyone's wife. I'm feeling very uncomfortable with those thoughts and I'm afraid of talking about this subject. I think that informing my girlfriend of my inability to continue our relationship if she gets married is wrong, because I will be forcing her to make a choice between her partners: either end our relationship or reject her partner's eventual engagement.

You’re afraid of something that hasn’t happened yet. Try not to let your fear of the projected future color your present; it’s not worth much to you to get upset over the consequences of something that has not happened yet.

You say that you’re afraid to talk to your partner about this subject, but that’s what you need to do. It’s up to you to make a call for yourself that you don’t want to date anyone who is married, because of what marriage symbolizes to you. You can tell her, “I’d like to know where your thoughts are about marriage or eventually getting engaged to Pilanzer. Have you two talked about that? Do you see yourself marrying him someday?” If she seems to feel that the two of them are on the road to marriage, you can make the choice that you don’t want to date someone who’s moving toward marrying someone else.

You’re not forcing her to choose; you’re making a choice about your own behavior. You can decide not to date someone if their other relationship has a dynamic that makes you uncomfortable. Don’t frame it as “promise me you won’t marry him, or else I’ll leave you.” It’s “I have realized it’s not healthy or comfortable for me to be in a relationship with someone where marriage is part of the equation or future plans.”

It’s entirely possible that she could tell you “oh, no, Pilanzer and I totally don’t intend to get married,” and then in a few years that could change, and she could decide she wants to get engaged. Or, she could tell you “yes, he and I are thinking that if things continue as they are, we will eventually be married,” you’ll end the relationship, and then she doesn’t end up marrying him. No one can control or predict the future. Your job is to do your best with what you have and what you know now, in the present, not to try and prevent some hypothetical future thing from happening.

It may also be worth thinking about, reading about, and talking with her about your perception of what marriage means. You may have some assumptions or projections attached to the concept of marriage that are worth dismantling or examining. Is your perception of how marriage will change things accurate? If your partner were to get engaged, would she experience it as “declaring her personal choice of a better partner,” or are you reading a message there that isn’t being sent? Does she expect that whatever life-partnership and societal-benefits marriage confers will also, eventually, be shared with you, once your separate relationship with her gets to a point where you two want to be so committed?

In the end, though, only you get to decide what kind of relationship is healthy for you. But you need to have enough information to make an informed choice. This requires having an open conversation with your partner, asking her for help understanding her intentions and expectations for the future and how she frames and understands her multiple relationships. Once you’ve had that conversation, you have to make your decision. It’s not forcing her to choose; and it’s not definitively protecting yourself against any future you won’t like. It’s you taking whatever path seems right for you based on what you know right now.