I've written on this topic before. Since writing that post nearly two years ago about my fear of extending our nuclear family with the meta connections of polyamory, most of those connections have collapsed. Back then, I was concerned about the effect on Small, but she is fine. This time, I'm more selfishly thinking of myself. (I can do that sometimes, right?)
Although that post was about 'chosen family', I didn't choose any of my metamours - my partners did. With only trivial exceptions, I've always been delighted by their choices. Despite some bad past experiences, metamours, semi-metamours (metafuckbuddies?) and potential metamours are still one of my favourite things about poly. I want my partners to keep dating, because I want to meet their dates! If they find someone they want to be around, the chances are that I'll want their company too. Everyone wins!
But I'm not a part of their relationships, and I don't get a vote in how they are conducted, or how long they last. They are relationships of choice, but not mine.
That's not to say that metamours can't become friends or even family in their own right, but I suspect it happens less often than we hope. When the supporting meta-ness of the relationship has gone, there will at least be change, and what comes out the other side won't be like the metamour relationship you started with. If there are sides to be taken, the chances are you won't be on theirs any more. People that I once felt intimately close to have ended up feeling like near strangers, and none of it was in my control.
The relationship between metamours and the way that this builds poly networks or 'polycules', is seductive. It is so tempting to call these connections family far far earlier than the connections deserve. I'm pretty sure that I'm not the only one to fall into this trap. Most of the time, these connections feel stronger than they are. One crucial difference between biological family and chosen family is, I think, that one is opt-out, and the other is opt-in. To put it another way, you don't need a reason to include your parents or siblings in your family, but if you cut one or more of them out, you probably had a damn good reason for doing so. Relationships that started for a reason, on the other hand, are more vulnerable to change. These relationships of choice can be stronger, but the former is more plastic. The bonds between metamours can appear deceptively like family relationships (because you didn't choose them), but like your sister's partner or your uncle's new spouse, they are as brittle as romantic relationships themselves.
This is not to say that we don't believe in the 'poly tribe' any more, or that we don't think that metamours can become family. We do. We're just more cautious about whom we include. Marge Piercy's vicious poem A Snarl For Loose Friends ends with the lines 'Don't count your friends by their buttons until you have seen them pushed a few times' and I think that's good advice for anyone you want to rely on. My relationships with my partners and their meta-relationship with each other have all seen multiple buttons pushed, and we're still standing. Our relationships have been proven strong, and so we feel safe to call each other family.
Poly people talk about communication a lot. But we've learned from past experiences, and now we trust the results of this button-pushing more than what is said. That's not just because they may be unreliable, but because sadly, relationships don't succeed just because we want them to.
Small is old enough now to talk about her long term memories. She talks about my boyfriend when he isn't here, as she does with the rest of her family. When she was with my husband's girlfriend this weekend, she asked about a trip they took last Autumn, several visits ago. If someone has stuck around, through thick and thin, even when things got hard, then they'll be around long enough for her to develop a real connection with them.
Me? I'm cautious, but still hopeful. I've seen it fail, but I've seen it work, and I'm trying to learn from that. The two men in my life have loved each other as well as me for years now. No matter what else we might have got wrong, this bit we've got spectacularly right.