Another suitable term would be "framework."
In other words, your loving relationship and trust in one another is a good foundation to build upon. But your conversations of non-monogamy, and the secrets you've both kept from one another give you a solid idea of where your current walls and doors are, like the framework in a partially-built house. But you can still say "You know what, I don't like that wall there, I want to tear it down. And I want to put in more windows to let in more light." (that would be an analogy to being more open and honest with each other)
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."