I would see a triangle with complete monogamy on one tip, polyfidelity
on one tip, and strich strangers-only swinging on one tip. (This
would leave out cheating and dishonest relationships in general.) One
side would essentially measure the relationship's openness to
additional love, and one side the relationship's openness to aditional
sex.
Keep in mind there are also asexual people and others who prefer to not
have this type of relationship. Just to make things more confusing!
Desmond Ravenstone