OP here. Okay. Yes, it is completely accurate that I would like to explore poly , in a limited way (ie. with Mike) and am mortified at the thought of my husband being with anybody but me.
That being said, I did tell him, and he was okay with it. I have since seen Mike again, and was honest about that, too. Mike has said that both his girlfriends have expressed an interest in me. I talked to my husband about that, too, and I think I would be okay with him exploring too if I'm part of the exploration.
Toward that end, we're thinking of getting to know them better as a couple, but I don't know where the secondary girlfriend fits in.
Mike and my husband respect each other. This could be interesting if we could figure it out.
We're being honest with each other.