Never is a long time. If you truly believe your husband will never entertain the idea, you do indeed have a lot of introspection ahead of you. That's a tremendous choice you would have to make. A soul-searching question would be: Am I truly the polyamorous sort, and really wish to share my life with multiple love interests, or I am dissatisfied with my current marriage and wish to get out of it? Be very honest with yourself. I know, it's not always so easy.
Here's another perspective. If you separated from your husband, and eventually divorced, and established a new primary relationship, would you still want to have other romantic relationships in addition to your new primary?
Have you and your hubby pursued marital counselling? Sometimes working on your marriage can improve communication to the point where things like this can be discussed in a non-threatening manner, so I wouldn't necessarily totally give up on your husband ever being willing to talk about this.