Welcome to the forums.
Well hopefully you are exaggerating on the "immediate divorce" comment because if that is truly an issue, there are other problems to be dealt with first. You also mention that you are a "strong couple together", does that mean that if you or he went away for a week or even just over night there are problems?
I broached the subject with my husband by joking about wanting a harem (of men), but that fit with our situation and may not work in your case. Chances are that unless he has been harboring similar desires, he will feel slightly hurt and insecure. Then you have to reassure him how much you love him and want to spend your life with him and your family and this poly desire has nothing to do with how you feel about him. After that depends on his reaction.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.