-Our relationship needs to be stable if we are interacting with others.
Yes! It will only be further destabilizing to you, AND hurt them, if you engage with others when you're not secure in your primary partnership, with good communication, a good solid trust and respect, and shared values and vision.
-I believe that because we have children who are depending on us to maintain a stable family and home, my boyfriend and I need to be the primary relationship if we are going to live a poly lifestyle.
Sure! This is totally valid, as long as you recognize that this means that it would be unfair to ask any new partner to be exclusive with you, and as long as you're completely upfront with any new partner that you're only offering a secondary relationship.
-We need to be a package deal and start out with only engaging with others together.
Really, really bad idea. You've had this go wrong twice now, in different ways. Trying to force yourselves and any new potential partners to adhere *more* tightly to a mold that isn't working is a bad idea. Here are some writings on why:
What it comes down to is that it simply doesn't work. Relationships... real, authentic relationships... build in different ways, at different rates, and you can't force an expectation of triad-ness on someone. Vees really are ok, as long as you're not *expecting* a triad, and thus setting yourself up for disappointment, or putting bans on the very natural act of dyad affection, and thus setting yourselves up for failure, or living together prematurely, and thus having to be around displays of affection that you're not yet comfortable being privy to.
-Cohabiting with a girlfriend must wait until the relationship has made it through a nice long trial period (1+ years, longer if they have children)
Very, very good idea. But really, I've gotta say, if you've decided that you guys are "the primary relationship", which I take to mean that you're not looking for co-primaries, why would you consider putting someone in the position of living with you at all? Or maybe I misunderstood you on that?