Should I talk to him about it or drop it?

My boyfriend and I have been together for a while and have had threesomes from time to time. We both enjoy it and are very comfortable with the whole situation and have clear boundaries set in place. My dilemma is I would like to be even more open and see (maybe even date) girls on the side? It wouldn't be one sided, I think with appropriate boundaries for each of us we could have side relationships. I personally feel that humans are not meant to be monogamous and I want and have so much love to give. I don't think he will go for it, in fact I think he might be offended as he doesn't see love in this light. He's a little old fashioned in life but adventurous sexually. How do I tell him, or do I forget it and enjoy just each other?