How do you respond when your husband accuses you of going out at night and having affairs? How can I validate?

From people that I have talked to in support groups and just in conversation this seems to be more common with Parkinson's Dementia than other forms of dementia.
You can not change his mind. If he believes that you are going out at night that is the reality in his mind.
Reassure him that you love him. You have been in the house the entire time. But if he still insists about the only thing you can do is walk away. Don't argue, it is not going to change his mind.

There are medications that can help with some of the anxiety and hallucinations.
I would make sure his doctor has correctly diagnosed the type of dementia. Lewy Body Dementia is one that some typical medications can not be used.
LBD is also a dementia where violence is more common. If at any time it becomes dangerous for you, or anyone in the house you may have to consider placing him in a facility for your own safety.

Be sure to be treated by at least a neurologist or better yet a neurologist who is a Movement Disorder Specialist. Contact the Parkinson's Foundation at 1 800 4 PDINFO. Ask for the info booklets, especially the ones on mood disorders and psychosis and caregiving. Very helpful information.

Unfortunately, with dementia, you are not going to be able to convince him of anything reasonable or logical. I agree with Alva to look into a med change that could help with this. If his hallucinations can be controlled or his anxiety or whatever it is that is making him accuse you of such things, that would be really helpful.

If I understand your profile, you are both living with your daughter? How long have you been there? How is it working out?

There are medications to help with Parkinson's delusions and hallucinations (which often manifest as just this complaint). Do know they have their side effects and that they can often worsen balance issues. It is a true catch-22 at times, with those medications that can help with balance actually increasing delusions. See the doctor regarding this would be my advice. May need to work on balancing some medications.