I don`t even see how it is 'friends with benefits' if you get no alone time ! None of my FWB`s ever tried to keep their spouse in the mix.
There is definitely a control/trust issue at play. To each their own, but I have never worried about 'what' someone is doing, but more so 'how' they do it.
I`ve known spouses that need to meet, spouses that need to converse, some right away, some later on. All that is fine. Many times, it can feel quite natural.
There does seem to be a 'creep factor' with some who seem desperate to meet, follow along, and keep an eye on things.
As a friend, I would tell him honestly, what puts you off,...Then let them chew on it awhile, and see if they change for the next person, or at least understand they need to be upfront about how things work for them. I think that is the part that bugs me most. It seems underhanded.