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I would just tell your husband that you have been thinking about poly, and that you think it might be good for the both of you. Maybe he will have "a reaction" to it at first, but you could still bring it up again later (e.g., "I've still been thinking about it, and wonder if we could just talk about it"). Just emphasize to him whenever you can that you want to stay with him and just do whatever is the best for the two of you.
Sometimes it takes time to get used to things. I would also emphasize to him that it's the honesty that's important to you (and that you want to know how he really feels, not just want he thinks you'd like to hear).
I hope you guys can work something out.
Love means never having to say, "Put down that meat cleaver!"