I have been thinking on this... and I think where I see the split is on level of commitment.
When I date, if I date multiple people... my commitment to them is... surface level. I am dating, thats the feeling out stage of a relationship. Are we intellectually and emotionally compatible. Are we sexually compatible. Are you going to be a total biatch in 3 months.
Poly is the "in a relationship" stage I suppose. You know you like each other, maybe even love each other, and you hit a certain level of sincere commitment.