My late wife and I had only one rule, that we wouldn't try to "make love" with anybody else.
You just reminded me of something... When me and Mr G 'make love' we always say to each other afterwards 'we can't share this with other people, this is ours!'. Each couple have their own 'thing' when they are alone together assuming that they are in love...
We have a sadomasochistic side in our love making, Fisting (vaginal and anal), toys and humiliation mainly which we love together but it's something that we don't take into the scene with us. I don't know as it's a rule but just something we keep for ourselves and don't share with people we play with. It doesn't feel like a rule and like something we have to be conscious of, it just feels right and obvious to both of us.
BDSM, D/s couples contact us and say that they're looking for x,y,z but we always say 'No thanks, we're not looking for that'.