this is a tricky one. hmmm.... you aren't getting what you want out of relationship one, but can have a world more out of relationship two... have a primary, but will not be able to add to that.
I honestly think I would go for it. As long as she knows that eventually you are going to require more and that it could end then if she is not willing to look at that, then why not! After all, it is quite often a good idea to work on a primary relationship first before searching for more anyways.
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