It is true that you would have to encounter the situation before you could truly know what your response would be.
My spouse had a heart attack at age 37, I was 27. His medication had a side effect, trouble getting and maintaining an erection. That was 14 years ago, he no longer takes the medication, but the damage was already done. (He can't take Viagra, it could kill him instantly.)
We have a really "good" friend in another state aware of our situation. My husband actually encouraged me to have sex with this other man. I eventually did, but not before a lot of conversations between my spouse and I. Sometimes hubby participates, other times he watches, and still other times he sends me on my merry way and tells me to enjoy.
This has worked for us for many years, but I don't know how well it would work for someone else. Most of all it takes a deep love, commitment and understanding from both partners.
My suggestion is to talk about the possibilities now. That "someday" just might be tomorrow.