Maybe one spouse was spitefully cutting off sex. Maybe the other was just unfuckable, whether due to hygiene or personality.

Well then they could divorce. I assume the period would be long enough (months or years, not days or weeks) that if someone felt their spouse "unfuckable" they could initiate divorce proceedings

What if she agreed to the act itself, to avoid being penalized in a divorce, but deliberately made it as unsexy and not fun as possible. Do you want a judge trying to determine what counts as sex?

I think that's been legislated pretty thoroughly (it's my understanding as part of having rape laws that entails defining what sex is) and would be laid out in the agreement. I don't think quality is something that would be laid out, just that sex must occur.

How would evidence about this get presented in court? Judges having to listen to he said vs she said about their sex Life with the underlying motivation being financial gain at the expense of the other?

I assume one party would allege a dead bedroom and the other would honor the law and not lie to the court? I assume counsel would advise their client not to lie about the amount or frequency of sexual activity?