From your posts I've gathered that you're not sure how you can have all that is expected in monogamous relationships (moving in together, having kids, getting married, growing old, moving geographical location together) in a poly relationship. (Am I right?)
I can definately relate to these worries. I'm still fairly young and have always had the 'traditional' relationship ideals, moving in, marriage, children etc. And I find that since I've been in a poly relationship, I worry about how possible these things are. As far as moving in goes, in monogamous relationships you're moving in with that one person and get a feel of what it's like to live alone with them, if they have another partner, I guess it would be different. The dynamic of it wouldn't be quite the same and I find that becuase the traditional dynamic is what I had always wanted, that sort of dynamic scares me.
I also worry about what will happen if one member of a polyship needs to move away, what happens? Does everyone move? Everyone has different things weighing them down to a location (kids, family, friends, jobs, etc), so
if one person needs to move for whatever reason, who do they move with?
But in the end you've just got to let things play out. As mentioned in previous posts, the realistic is not always the only possibility. There's nothing wrong with having these worries, in many ways they're justified (it can be difficult enough reaching a compatible dynamic between just 2 people) but that doesn't mean it can't or won't work out. It's possible that a few years down the road, you, your boyfriend and his wife could be living together happily and that you will recieve all of the 'forever' things that you need. It is also possible you won't but if you really matter to each other, and if everyone involved is willing to put in the effort, it's worth a try.