I think it comes down to the attitude of the spouse. As peeples stated "anything short of a coma" there are ways for your mate to creatively please you. If they don't think you are worth the effort then they won't make the effort.
There are some people that would allow their mate to have sex with other people as long as they are discreet. In those cases that would not be "cheating".
Having said that if I was with someone that did not think I was the effort to attempt to please and they insisted I did not have sex with anyone else then I'd probably leave. I know (myself) well enough to know if I stayed I would probably end up cheating on them.
Personally speaking if I was not able to perform sexually I would use every working body part I had to please my wife. I could never adopt the selfish mindset of: because she can't give me orgasms then I won't give them to her.
There is a difference between being ill and being dead!