What I'd like to know is... what happened 5 years ago? Are you saying that your sex life was good for 25 years and now suddenly it has stopped-- or at least-- stopped being good?
It sounds like having a happy sex life is important to you. Does she know that? Is having a happy sex life (for both of you) important to her?
Perhaps the way to not hurt her feelings is to find out about just how she does feel about sexuality. What are her dreams of the perfect sex life with a partner? Is that something that she'd be willing to discuss with you?
When you ask her this, the question is not just what a great sexual relationship would look like to her, but what it would mean to her? How did she develop these ideas over time?
Then, if she's open to it, you can tell her your dreams for a great sex life. But I have to agree with the other poster-- telling her you'd like to include other people is probably way not worth the risk, unless she has already expressed a similar desire to you.
If she's unreceptive to having any discussion of what happy sex life looks like and means to her, then I would suggest dropping it for now.
Perhaps the two you could trade stories of other aspects of your relationship-- trading stories of your dream/vision for the relationship.
The idea is not to agree, but simply to get an open discussion going. Each one gets to talk for a bit without interruption. If you start on more familiar topics and wait a bit, maybe that will make the happy sexual relationship dream discussion easier.
Eventually, though, it will be necessary for you to let her know that your need to have your sex life (with her-- don't even hint about others)to be more varied and interesting. And through out all of these, make sure that she hears it a lot that you love her very much and find her attractive.
Tina Miller, M.A.