I can understand his missing her and being lonely, however, she was ill and it's probable she did not have the capability of consenting.
I don't think that you're understanding my point. It's one thing to hear, from a doctor, "your husband/wife has Alzheimer's and is slowly but surely losing mental capacities." It's another thing to see it for yourself, understand it, and translate that into life change.
For example, my grandfather pretty much made all of the important decisions when he was in his right mind. But well after he was diagnosed, my grandmother would still run everything by him, even though he was "out of it," so to speak. She even asked him what he thought of being put on hospice care. He was, of course, in no position to be making important decisions. But that's how they'd always lived. That's what she'd always done.
Hearing a doctor say "look, bla bla bla" isn't enough to overcome a habitual attitude towards a spouse that you have interacted with in a certain way for decades.
Suppose that she wasn't able to consent, and suppose she hadn't previously (in more or less her right state of mind) expressed her wishes in advance, we can't automatically assume that he's a sleezy scumbag rapist who deserves to rot in prison.
Chances are, he just saw himself doing what he did with his wife non-stop for decades, up to and including the early stages of her disease. "But the doctors said..." Sure. Doctors say a lot of things. What's your point?
And consider it from this point of view: Suppose, had she been in her right state of mind, someone had asked her: "In these conditions, you not being in your right state of mine, and your husband having done such and such a thing, would you want him incarcerated for rape," what do you think she would say? Be honest.
The OP title is trying to spin this as a romantic, honorable event. Was his love and support only offered under the condition of sex?
I wasn't making that point. I'm simply pointing out that we shouldn't be so quick to judge the man, and I think that it's probably extremely unjust that he's in prison.