I would just say to your partner -- "Of everyone you could get involved with this, she will be the hardest for me to handle. I know you're attracted to her, but it would mean so much to me if you didn't pursue it. I don't want to limit you, but I'm just seriously worried about what this will do to our relationship. That said, if you feel like you really have to give it a go, I'll do my best to be ok."
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.