Are you partnered or solo? It should be easier of you are solo.
I wonder if you are building up some apprehension in your mind about "dropping the poly bomb" on people, making it some big deal that you are going to reveal to them like a deep dark secret instead of just another aspect of yourself that you talk about when getting to know someone. People will pick up on that kind of nervousness.
It doesn't have to be such a heavy topic. Be casual and matter-of-fact about it. You're dating, not looking for exclusivity, and hope to see this person again, blablabla.
Hot chick in the city.
Oh, oh, can't you see? Love is the drug for me.