It is only prohibited by law in the case of marriage not relationships. Also, it is quite possible to have several partners in a real life polyamorous "harem". A lot of people have started entering this sort of lifestyle. I personally have had this with a set of twins and their best friend. Typically what happens is one of the people involved is not as emotionally mature as the rest and they get jealous so the relationships have to change or end. I have some friends that the dynamic is 1 guy /w 3 girls and they have been solid in that 4 way relationship for about 4 yrs now, so yes it can and does happen but it isn't discussed much openly due to the sexually and emotionally limited views of the average person.