Broken crayons still color.
You know that it's difficult for people to understand all the "technicalities" of infidelity if they've never walked the path. I say cut the guy some slack. He's on the outside looking in, only knowing what his wife has said of the situation.
And let's be honest. Your ex-boyfriend has turned into a jerk. Seriously. He's a cruel, selfish, a-hole. Does it condone your cheating? Absolutely not. However, he's still a cruel, selfish, a-hole. If I was looking from the outside in, seeing my wife's BFF dealing with an abusive jerk, I too would probably think that you'd be better off with the AP. Because after all, I wouldn't know that broken attracts broken, and everything else we learn going thru the University of Infidelity.
I don't think his statement is a massive red flag that means he's going to inevitably going to cheat on his wife. I think he's (somewhat ignorantly) trying to find a safer, better solution for the situation you're in.
"Courage is being scared to death and saddling up anyway." - J. Wayne