OK.... I know I'm getting ahead of myself but you will also need to discuss safe sex with your potential other and your husband. He's mentioned flings and in essence your husband will be sleeping with everyone he does as will you. These are the gritty details of non - monogamy of any kind.
As far as how to broach it... I'm sure others will have insight. There is also a lot of threads on here about that already.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over