Sometimes people who consider themselves poly, have every freedom to date, do cheat, and lie about it to their poly partner(s). I am reminded of our longtime member here, BaggagePatrol, and her husband of 5 years, Elemental. He would often cheat and lie about it, even though they had an open relationship. Some people just seem to get off on the sneaking around. It adds spice for them. Sure, it's a leftover from so-called monogamous, but cheating days. Hard habit to break. BP chose to divorce her cheating husband and seems to have found some better partners, though she is still grieving the breach of trust. She has a blog here that goes over the situation exhaustively.
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, former swinger, 42, M) since Oct 2015
"Master," (mono, 34), miss pixi's Dom for 2 years