I would take anything your fiancee does or says right now with a grain of salt. Being poly can sometimes feel like a rollercoaster. You can be fine with it, even turned on by it, one day, and overcome with horrific jealousy the next day.
Have you 2 been mono since you met? It sounds like it. Poly was more theoretical until now?
Unfortunately it's quite common to be poly for oneself, but then feel raging jealousy when one's partner gets interested in someone else.
If your gf is busy raising kids, maybe working full time as well, and doesnt have time or interest in dating others right now, good for her. But to expect YOU to not date just because she isn't, smacks of possessiveness.
Give her a few days to simmer down. Don't talk more about your new potential partner right now. Sometimes our hearts have their own reactions to things, that are in conflict with our rational sides. Love her up, take her out, watch the kids so she can go do something nice for herself. She needs reassurance.
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
There's no lying in polyamory!
I'm a 58 year old woman with 2 partners:
miss pixi, my live-in gf, 36 (together since Jan '09)
Ginger, bf, 61, married, lives nearby (together since Jan '12)