I would equate slut more with sleeping with whoever whenever they want without thought of emotion or real connection. The people that I know that live poly lifestyles have multiple partners in the emotional sense and more often than not keep the sexual activity within those.

On the other hand..'slut' is a standard set by an outside group so I don't know why anybody should really care if some stranger thinks you are a slut for being in love with more than one adult.