while ideas (mine first of all) are getting clearer and clearer through important posts going on here, i felt in jumping in to add:
there are some couples (don't know what about any poly, actually) who begin a marriage/union fixing a determined period to their relationship; for instance, one year plus one day: on that day they will "decide" if to renew their union or to split. every couple chose the length of their first/sec/third/etc. steps relationship.
do you think you would make someone happier with this?
can you imagine some results?