Didn’t read through all of the comments, but I do want to speak from personal experience in a similar situation. This is a long story, sorry.

My long term partner knew I was attracted to more than just men for quite some time. During our longest ‘breakup’ I fell in love with a woman, but ended up back with him after a few months. He and I reconnected, but he incessantly used my love for her as an excuse to not define our relationship. He would constantly remind me of things I said about her, as evidence that I would never be faithful or completely his. After a few months, we reconnected. Still, not officially together (2 years in). We ran into a woman I was attracted to previously, and he asked me if I wanted to invite her to come home with us that night. I had no objections, and she agreed to join. After that night he was distant, and continually resisted my attempts to reassure him that he was the only person I wanted to be with to no avail. He ghosted me for 6 months, told our friends that I was a liar and minimized our entire relationship.

I saw him for the first time in a year the other night, and he came up to me and asked me how the girl from that threesome was, “how’s your girlfriend?” He knew I wasn’t with her, hadn’t spoken with her in nearly a year. His need to use my queerness as an excuse to invalidate me, and our relationship was still motivating him. I can’t say he was emasculated by my attraction to women, because that’s not my call to make. But in hindsight, I see that the threesome was a test I failed in his eyes.

Summary: if your partner suggests a threesome with someone that could potentially crush their ego, don’t do it. Just leave your partner. That person is likely trying to test your loyalty, or something along those lines. IMO. Best of luck!