It doesn't have to be a heavy conversation on the the first date. In the "getting to know you" phase something simple like, "I'm not looking for exclusive relationships right now" could work. Then, if you get closer to someone or if they have questions, you can go into your reasons for exploring poly. Waiting until you're in a committed mono relationship isn't fair to your partner- you'll have basically created a relationship with them under false pretenses by lying about who you are/might be.
I would be careful about those best friends who scorn your poly friends, though. That might be a good place to START talking about reasons for being poly, as an academic exercise rather than applying it specifically to you. If they just can't accept it at all, they're unlikely to be very good friends to you through your journey. There are all kinds of reasons to hide being poly from certain family, co-workers, and society at large, but if you can't be yourself around your friends and talk to them about the things you're thinking and working through, can you really call them your friends??
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack