I Don't Know Where To Turn
Looks like your husband was more "experimental" , or at least to seriously discussing it, until the kids arrived. I think part of that may be now with the added responsibilities of children, he may feel he has more to "lose" by taking a chance on any kind of non-monogomous relationship. There is no "normal" or 'not normal".
I agree with those that say you should have therapy because if you feel strongly and continue to be unfilled, eventually you are going to cheat in one manner or another. You may find that you are actually able to live monogomously, or you may not.
If you keep pushing your husband too much, he is going to get defensive, followed by suspicious, and it can go downhill from there.
Nothing wrong with discussions about it, IF he wants to discuss. A lot of men think they can handle the wife or partner with other, and find when the real world happens that they cannot handle it. From what you describe, it might be easier to start your husband off with talk about "swinging" first, because that might make it less threatening if you establish firm ground rules about no emotional commimttment, which if the children have caused his gravitation to monogomy, might be causing his reluctance to even seriously talk about it now.