I would say thats a one sided view. Relationships are a tonne of work. Poly relationships are not any harder than a mono one, it just happens to end up that way with all the communication and timing it takes to make stuff work. Of course that is in general.
Does the mono work harder in a poly relationship than the poly in the same relationship. I don' t believe so. depending on the situation, there is a lot of restrictions and communication required by the poly to help the mono. As hard as the mono is working to understand, the poly is working to help...or should be.
...that I suppose is the caveat.
There are seemingly two type of poly people (generalizing here) I ... don't look for anything. I leave myself open to the chance of finding people. I am also very social, so I suppose it appears I am always looking...when in fact I just like to keep myself exposed to the possibility
I enjoy the dating but i don't fall in love with everyone I date
My wife on the other hand is content with playing the waiting game, seemingly letting situations fall into her lap. I believe this also has to do with how active I am...
It also isn't a question of contentment. I am content with my wife...but I have more to give, I enjoy loving people...will I run out of love to give...not sure..I just don't feel monogamous, I can't fathom not being available to the ability to be with someone else if it comes along...(ps I don't use love, because I am in an open relationship with the ability to fall in love, i don't restrict my openess to love only)