If your new gf is in a relationship and having sex with you while being engaged to a guy who doesn't know about you, this is not polyamory. It's cheating.
Polyamory means ETHICAL non-monogamy. Everyone has to know and agree to their partners having other partners.
If you want to be poly, you must not be in touch with her again until she tells her fiance. If you don't care, and just continue to see her, you're complicit in her cheating on and lying to the man she plans to marry. Not a good way to start married life, imo.
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
"Master," (mono, 34), miss pixi's Dom for 2 years