Married couples should get 3 open relationship nuts, like 3 hearts in Zelda. You can either use those 3 nuts to bang three women (or be banged by three men), to bang the same person 3 times, etc.
Set it up like insurance. So at the end of the calender year your nuts are renewed. However you cannot stack nuts, ie not use 3 nuts in 2012, thus giving you 6 nuts in 2013. Nah, all that you don't use you lose.