I would say that if your marriage really comes first, then a "low maintenance fun fuck" who has a history of threatening your marriage and who makes your spouse miserable shouldn't be an option. There are plenty of other women out there, and surely out of all of them there is another low maintenance woman who is fun to fuck, so why pick one who will hurt your partner and might even damage your marriage? It seems like he's not really exercising good judgment here, or else his priorities aren't really what he says they are (I would assume the former). I'd ask him to just forget about it.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.