Sounds very familiar.
My advice-just dont jump the gun and act on anything.
Start the topic without personal examples or anticdotes. Maybe start with something about Will Smith and Jaeda Pinkett (happy open marriage) or some other big name whose out as happily married and poly. I know there was a singer who came out poly... I can't remember her name. I think her husband was an author.....
"Love As Thou Wilt"