I'm coming out of lurkdom to suggest that, while hurting people's feelings is not something anybody wants to do, being honest is really important. And being honest is probably kinder in the long run.
You strongly feel that marriages are more sacred and special than other relationships. And that feeling isn't right or wrong. It just is. And you feel (if I've understood it right) that you can only be married to one person. Again, there's no wrong or right about it. It's just how you feel.
But it's fair to be honest about this with your girlfriend, so that she may seek out a primary relationship of her own. She can seek out somebody who will be able to share that sacred bond with her and can co-parent with her. And she can still be in a relationship with you and your husband. Being poly means you don't have to pick one over the other.