I’m in a relationship with a woman who’s poly while I’m not. Right now, I’m the only one she has and she can’t promise I’ll always be her only one, and she’s also said it’s not likely I’m the one she’ll choose to marry someday. However, she swears I’m the one she’ll always love most by far, and that I’m absolutely everything to her, wanting to keep me for life, even if she does have other relationships and marries someone else someday. How can this make sense, and can I believe her?

I think it’s healthier to let relationships unfold organically and be what they need to be at the time. It sounds like she’s doing the opposite by focusing on future contingencies that don’t need to be addressed right now. It sounds like trying to predict/pre-define how everything will shake down between you two “for life” or “someday” is preventing you from dealing with how things are right here, right now - which is the only thing you can ever really address.

I don’t see the purpose in telling the person you’re with that they’re “not likely” to be the one you “choose” to marry - that seems manipulative and rather cruel. Perhaps part of the issue is getting to her understanding of what marriage means. Why is she so concerned about picking one person to marry, and why does she feel like that’s a distinction that’s so important to make between partners? Talking about that might help - but I think there’s a deeper issue here, and that’s the way she’s treating you.

There needs to be some clear boundary setting between you two, so both of you agree on and understand the terms of your relationship. It’s not about promises, it’s not about the intensity of feeling - it’s about realizing what needs you two have and how/whether you can meet them for each other. If she keeps deflecting from practical, productive work on your relationship as it is now by using sweeping emotional promises and manipulative hints at a currently-nonexistent future partner, then it’s not a healthy relationship.