I guess I am a hardcore practitioner of poly... I am in 2 relationships and have been in up to 4 real life relationships more than once over the past 13 years. But I have been inwardly bi/pansexual and "poly" ever since I was a teenager.
I am not shunning you, Ry. I applaud you taking a big step back to assess how you managed your poly practices in the past.
Especially since you have 2 very young children and you're moving thousands of miles away very soon. I imagine your ability to be present for both Matt and Si degraded even further when your babies came along. I am not sure if you are also currently employed outside the home, but even if you aren't, romantic/emotional/sexual relationships with 2 partners, plus 2 very young children, mixed in with what sounds like extremely poor communication skills, makes for a very full plate, and an unpalatable meal.
The last thing you need right now, imo, is another secondary partner. I support you in playing with your children, attempting to establish trust with Matt, packing your gear and moving from London to Australia, getting used to your new city, supporting your daughter's adjustment to her new school, unpacking and decorating your new home...
You are probably poly in your heart, but so what? You've got shit to do.
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, 60) loving miss pixi (poly, F, 38) since January 2009, living together since 2013
also loving Punk (monogamish, 42, M) since Oct 2015
"Master," (mono, 34), miss pixi's Dom for 2 years