Rita, one thing I've come across in my thought paths, that really helped me was this. While I was thinking that if another came in the picture, I kept feeling like it would be more or less a copy of what Jen and I have. That is not the case, and cannot be the case. What you and your hubby have is irreplaceable. The love that you share is immense. If anything, others that come in will be more of a supplementary relationship.
So don't think of it in terms of black and white, since it's pretty much never like that. It is sort of like friends. Different friends fill different roles in your life. I know it's not the same, but the concept is in line.