Some more details:
Based on the above distinction , there can be also all possible combinations of these elementary types, therefore in total 7 types of intimate relationships.
The polyamory mode of intimate relationships, is directly compatible, with 1) The physical sex type of intimate relationships and 3) Long term partnership. But it is not compatible with the 2nd type of intimate relationships, that is falling in love. Falling in love is a special case that demands for the monogamic mode, as it is like being emotionally pregnant. But it does not last very long (say 1 year in the average) and thus the restriction to the monogamic mode is not a severe restriction of freedom, while at the same time, refraining of having sex with other persons, when you know that your partner has fallen in love with you is a significant respect of the emotions of the other person.
Also according the psychologist of marriages J.M. Gottman,(see e.g. by J.M. Gottman, J.M. Murray C. C. Swanson, R. Tyson, K.R. Swanson, http://www.amazon.com/Mathematics-Marriage-Dynamic-Nonlinear-Bradford/dp/0262572303/ref=sr_1_1?s=books&ie=UTF8&qid=1385038485&sr=1-1&keywords=The+mathematics+of+marriage)
there are three types of marriages, or couples relations.
1) The Domination type.
2) The Competition type
3) The Cooperation type.
In the domination type on of the two is dominating the other. The positive emotions of the dominator may create negative emotions to the dominated. But the positive emotions of the dominated do not create negative emotions to the dominating (win-lose). Such marriages may be stable but usually unhappy, and sometimes with violence.
In the competition type, the positive emotions of each usually create negative emotions to the other, and vice-versa (lose-lose). Such relations or marriages lead very fast to divorce.
In the cooperation type , the positive emotions of each create positive emotions to the other and vice-versa (win-win). Such marriages are not only stable but also happy. Usually they start with sex, pass, to falling in love, and result to friendship and love. Each one has space in his/herself for the self of the other as if he/she was his/her own self.
It seems that the statistics of couples after 5 to 10 years of couple-life show that
1) Group A: About 3% are agreed monogamy couples in choice of principles or values and by agreement, and also their practice is of the monogamist type. But from them only 1% is enjoying it as natural mutual love, while the 2% rather suppress themselves so as to act as monogamist, which often leads to verbal fights and nervousness.
2) Group B: About 17% are agreed polygamists, without hiding it, without saying lies to each other or complaining, without jealousy, and accusations when sex with persons outside the couple occurs. They believe that in this way, their relations is going to last longer, due to tolerance and discrete love. And it does last longer, than the normal.
3) Group C: About 40% of the couples have a polygamy practice which nevertheless one or both hide ,so they claim that their relations is monogamy. They live in hypocrisy lies and deception, because they believe that this is the only way their relation can last: By hiding that under the table it is polygamy, while on the table it is monogamy.
4) Group D: About 40% of the couples have a polygamy practice which nevertheless none of the two hides it, but still they believe only in agreed monogamy not in agreed polygamy and they hope that in the future their relation will be entirely monogamy.
It is obvious that most people would prefer to be in the Group A (and in particular in the lucky 1%).
But also most people are afraid that they will experience their relations as being in the group C or D. So they are afraid of falling in love.
The relations of the group B, last longer than those of groups C and D. And those of the group C last longer than those of the group D.
The most unhappy is the group D. At least consciously. Because their beliefs and expectations do not agree with the reality. But when a couple from the group C passes to the group D, the pain is maximum. The couples in group C, suffer secretly but they do not know it consciously Their psychic state is the most dangerous.
Not very many couples can feel comfortable being in the group B. But when they are, are genuinely happy, realistic, humorous and of help to other couples.
Most of the couples in groups C are from southern countries, or non-developed countries, while groups D and B is more advanced countries, and northern countries.
The most happy is of course this lucky 1% of the group A. Then group B, then the unlucky 2% of the group A. Then group C, and finally group D.
It seems that the dynamics that last 50 years are
1) The group A has been decreasing, and in particular the unlucky 2% of it.
2) The group C is decreasing ,
3) The group D is increasing
4) The group B is increasing.
Famous spiritual and wisdom teachers , like Rajneess-Osho who was love and happiness centered with their own ashrams where people could escape from the "jungle" of the survival mode of social life, usually advised couples in troubles to shift from groups C and D, to group B. Given the fact that the couples of the groups C,D, present in "face value" an ideal, that of monogamy, while their subconscious drives them to polygamy, thus their practice does not support the ideal, and it is plainly obvious that it is better to deal with their actions , desires and instinct, and accept themselves as they are, instead of beating themselves with a theoretical to them ideal, thus move to the group B.
While other teachers oriented to power, and control in life and to people that do live in the "jungle" of survival of social life usually advise couples in troubles to shift to the group A, even to the unlucky 2% of it, which sounds perfect advice but it is under question how much feasible it is.
Maybe, in another civilization, on another planet 3-4 thousand years more advanced, the percentages of group A would be very high. But on the current earthly civilization the reality seems to be close to the above statistics.