I should also add that I don't go out of my way to tell my husband all the details of what Mike and myself get up to but we always answer honestly when asked about what went on during a date.
How about NOT telling details even when he asks? Keeping boundaries? Because what he NEEDS to know are things like safe sex practices, expected time home, etc. You are his spouse -- there's a basic level of concern and consideration to meet so he can feel safe and so he can feel his spouse is safe.
He might WANT to know details like if you got up to spanking or not this time during sex share, but sharing those kinds of details is fueling his insecure thoughts and those kinds of details are not yours alone. They also belong to Mike.
It's ok to say "I see you would like to know that, but those are personal details I will not share. We did share sex. I will not detail it. Just like if Mike asked, I would not share sex details with him about (you + me)."
Other than that -- you are doing all you can do -- being honest, keeping agreements, being punctual, not having anything to hide, etc. Or so it seems to me.
Has he articulated what the fear is about? Besides that he feels
like he is being punished? What behavior are you doing that is "punishing" to him? Is he worried Mike is going to hurt you somehow?
Hang in there!