Why Would Any Female Allow Her Own partner to have sex with another woman & Still not even care about anything?

I have friends with benefits from a few years ago between 2010 which is now his own girlfriend told me she is allowing me to have intimacy with her man since they both are unable to spend time together even though she is a nurse. It even seemed odd which I never heard of any other female unless in an open relationship would be into sharing partners. Yet he keeps gaslighting me & disrespecting me in public yet If I ask him if any one else talked towards his own girlfriend like he does to me. What would you want to happen? He just walks away from me just foolishness with him.
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Many people while I am many times I would be in public stores like Walmart; I would be getting ridiculed, gas lit towards me. Continuing to talk down to me while we would be at a self checkout ; even at any stores people just would turning a blind eyes ; to how he treatment towards HIS Real living friend who is the only person who is the only friend after they all have passed away from addicting to the stuff.
Why Would Any Female Allow Her Own partner to have sex with another woman & Still not even care about anything?
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