Is it okay to cheat on a spouse that has Alzheimer's?

[Gnosis]

It's not like they'd remember it even if you got caught.

Or, more seriously, what sort of obligation do you have to a spouse that's so far gone they don't even remember who you are anymore? In other words, was it evil of Michael Schiavo to be living with his girlfriend for years while his wife, Terri Schiavo, was brain-dead, but physically alive, in the hospital?

Do they not recollect the wedding vows each made to the other?

“…to have and to hold from this day forward, for better or for worse, for richer, for poorer, in sickness and in health, to love and to cherish, from this day forward until death do us part?”

Within that sacred promise is found the steadfast answer to your question.