I understand the predicament of legally married couples, esp. those with kids. If they have been honest about their situation from the very start, non-primary poly partners should not feel bad 'closeted'. Being indiscreet will bring them a lot of legal/financial troubles.
In my own experience when I used to have a bf (which may be considered as non-primary), we came up with a work-around solution ("selected outing"), wherein the bf introduced me to his own circle of friends and family as his gf/partner so long as his circle was away from and there was no possiblity of interacting with my own circle of friends and family I share with my ltr primary partner (it can be considered primary because Im living with him). We used to live in a big city during that time, but honestly, i don't know how it will work out if we were in a smaller town.