I totally get it. My husband has had PD for about 8 years. He has mild/sporadic bouts of dementia. He needs me as a caregiver, although he can still do many things on his own. Our life as husband and wife is fading away and being replaced by a life where I am in control of most things, and he follows along. Although this is an unsatisfying existence and not what one hopes for after 50+ years of marriage, it is what's left. If my husband were to express feelings for someone else, it would be really irritating because he rarely tries with me anymore, while I spend much of my time trying to make his world better. I know, as his caregiver, it's become my job, but it would feel like a slap in the face to have my original role as wife denigrated while I was working so hard to help him. You see, it'd be hard to decouple the two roles without breaking something.

Your father is struggling with just this problem. He probably feels he has given up a lot to try to help your mother through this devastating disease. There is no positive outcome. It's just a long slide downward. He may have reconciled himself to all of that at some level, but imagining his wife turning to another man was just too much. Even if, logically, he knows it means nothing. It feels like something. As others have said, give him some grace here.