No-one ever follows up on these 'OMG I'm having problems!' threads
The shortest version is - problems ironed out, man and I are happier and stronger than ever, and joyfully poly.
The rather longer version is: turned out that C actually was looking for a pretty serious relationship; awesome though she was (and is - still a close friend) that wasn't something either of us were able to give her, together or separately. So we broke up after a few months of committed dating and a lot of negotiations over whether she wanted us to be a closed triad or not. I certainly learned a hell of a lot from that relationship, and I am grateful to her and the situation for giving me the kick I needed to really reevaluate some of my assumptions at the most basic level.
Since then, I've realised - as is a boringly common story! - that a lot of my panic was based in my own ingrained cultural assumptions; even though I felt like this was something I wanted, I found it hard to accept when it was actually happening.
One of the things, oddly, that's really helped is having come out to my family. Their overwhelming support and positivity has helped me see that this totally and absolutely can be part of my life, and I don't have to feel like it's a dirty secret that I'm having to keep - I can be proud of the choices I've made and the way my partner and I are forming our lives, and we can be proud of the other lovely people in our lives who are important to us.
I've come on such a journey in the past year; no doubt there are lots more bumps down the road, but I feel really strongly that - even if circumstances lead to taking a break - this is something that will always be part of my relationship with my partner, even if only as an option. And that now makes me happier than I can even explain. He and I are stronger than ever before; we've both made a lot of mistakes over the past year, but we've never made the same one twice, and we've kept talking and kept loving. Pus, the sheer joy I get from seeing him nuzzling up to lovely ladies is awesome, and I also have a long-term (male) lover from whom I am also learning a lot - which benefits my primary relationship too. So much love and joy.
Now I am less stressed about relationship stuff, my worries about babies have also faded somewhat. I know this is something he wants too, and me stressing over-much about it leaves him with no space in which to say 'oh, aren't babies adorable?' without me jumping on it - in the same way that him being slightly more reluctant leaves me no space to say 'you know, I am a bit worried about what this would mean for money, social life etc'. We'd both sort of carved out our corner and got stuck in it. It will happen, and if in a year he's still reluctant, I'll address that worry then, rather than borrowing troubles. It's not like he'll think my desires have just gone away, there's no danger of that
I thought you might appreciate a follow-up, especially as it's such a happy ending