Some thoughts on this:
Monogamy implies that we can have only one true match and that we are only capable of truly and fully loving one person, and there is a schema that two people marry and establish a unit.
Poly is a multischema practice. It may include a relatively small closed unit or a more freeflowing open unit, but basically accepts that we love more than one person with a special feeling for each person that is unique to the relationship with that person, but not detracting from the relationships with our other loves. Some loves may include sex, other loves may not yet be very intimate and there may yet be loves where it is more sexy than intimate emotionally and all are okay depending on one's emotional style and needs.