^^^I think you pretty much hit this right on the nose Timo. I have never had a 'long lasting'' relationship, but was raised in a montheistic society and home, with the standards of monogamy that accompany it.
To take this a step further anthropologically; I think the question is, can the person and their partner be one hundred percent honest, both physically and intellectually, to sustain and open and true relationship? That is, not only with themselves, but with whomever they are exchanging sexual partners with. If you are not open with your usual significant other, than the repercussions are likely to spill over in to the other two (or more) people's lives. Possibly even ruining them all. From both a romantic and social perspective.
If you were to do this, I think you would have to factor in not only the fact that your significant other will be copulating with another person and it is likely contrary to the social mores that you all were raised on, but it will almost certainly be against the grain of the social majority's feelings on the matter. If it were ever to become public knowledge, than the ramifications would be nearly, if not permanently irreparable.
Just my personal opinion on the matter.