Relationsships with several people have been around in a lot of cultures all over the world. Harems have existed through all time periods. The main problem in the past was ensuring paternity so it was usually one man with several women. The bible has several examples of men taking multiple women, in many islamic societies taking multiple wives is ok, in Nepal one woman could have several men.

Of course relationships have mostly been formed by economic necessities, not considerations of sexual preference. A woman might have several husbands in a place where living conditions are rough and it takes a lot of manpower for one household. A man may have many wives where men often die from wars and the women need to be taken care of. If these rules are kept for hundreds or even thousands of years, can you still call them a phase?

Besides you must not forget that the western view on history has been heavily shaped by christian values. Obvious proof that people were homo-/bisexual in the past was reinterpreted, the same goes for any form of open/poly relationship. “They were roommates” has become a meme for a reason.