Just to get some things straight.
Love Polygon and Harem are, in my view, fundamentally different and mutually exclusive.
An Harem is when various characters like the same character, having a romantic interest for the same character. So character A is liked by character B, C, D, etc. This is Rent-a-Girlfriend and a lot of other anime.
Love Polygon is different from this. Polygon is a plane figure made up of line segments connected to form a closed polygonal chain. This can be a triangle, a square and so on and so on (altough usually 5 or more lines). In this sense a love triangle is a love polygon. Different from the harem, a love polygon is when characters like different characters but those characters like other ones. As an example character A likes character B and character B likes character C and character C likes character A. This is love triangle in this example. Add one more and it would be love square or whatever, which would also be love polygon.