I guess you could just use the, "how do I think my partner would feel if they saw me" rule. Knowing your partner, try to imagine what they would be feeling if they were watching you with some one else. How would they react? How would you act differently if they were witnessing it? Would you be as close? Maybe it's just better if they don't see or know it if it obviously doesn't cross boundaries. Some people would be very sensitive to something that wouldn't phase you.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over