False accusations of adultery illustrate one of the most common kinds of delusions sometimes seen in the later stages of Alzheimer's. If you haven't experienced them, count your blessings; such delusional thoughts might never come up. But if they do, know that the quality of a couple's relationship has little to do with this particular delusion. It's more about insecurity and anxiety -- feelings that things are happening that can't be controlled (because they can't be explained, given lack of memory, reasoning, and judgment).
So skip the rational explanations or sharp protests. What to say instead:
Rely on humor: "Sorry, you're stuck with me, dear." Or, "I don't know why, but Mom insists you're mates for life."
Or empathize: "I can see you're upset about that. Who wouldn't be?" "You must find that unbelievable; it sure seems hard to believe."
Then distract: "Did you see how many birds are at the bird feeder today?" Change the subject or move to another room or location.