I'm deeply sorry for your pain.
Seems to me there are a few things going on here--
NRE (new relationship energy)
to breed or not to breed
You've only known her 6 months, so you're still in NREland. Yes, she seems great, all you've ever wanted in a woman, the hormones say. But wait! She wants kids, you don't. So, she's not exactly the perfect woman for you, after all. This is a huge stumbling block for those that are in the child-free mindset. Many a couple has broken up over this.
Also, they have the American dream going... A male, a female, a baby in the near future (gods willing). That is supposed to be a complete unit in this society. However, apparently she's polyamorous, and he's at least polysexual. Is this a phase, this dalliance with you? Will they forget all about having extra love/sex partners once a demanding baby, no time for dates, sleepless nights, etc., comes their way?
One thing I am harping on lately is, when you are poly it's (relatively) easy to fall in love. But actually nurturing and sustaining a relationship, that's the harder but crucial bit. It could be the issues you are having, it could be others. But the only way to work things out is to have many many open, painfully honest talks. If they don't feel they "need" to talk about their emotions like you do, guess what? If they want to keep you, they do need to. They need to hear you, they need to think deeply about what having you in their lives long term will look like. Having kids is hard. Being poly and having kids is even harder. Do a tag search for "kids/children and poly" here.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
also loving Punk (42, M) since Oct 2015 (he has recently downgraded us to friends)
"Master," (mono, 34), miss pixi's Dom for 2 years