Dont' use the word boyfriend though...

I would say

"If you want to have an affair with another woman's husband you do it OUTSIDE our home and NOT EXPOSE that to our kids"

I dunno... something like that... I don't like the idea of even setting that boundary because it sounds like validation that you accept the affair.

I would call OM's wife up and tell her he's calling your wife... Keep calling his wife... I woudln't be surprised if OM didn't like that...