Once again, open and honest communication would seem to be the key.
That's a stereotype.
People with personaliy disoders have no insight. To discuss means to make them angry. You keep discussing, they will keep getting angry. There can be no open communication as they have no insight. They are simply bad thinkers, even if they are genius otherwise. There are few people who meet diagnostic criteria, but many who have some of the characteristics. Then the question is if these characteristics are weak or strong.
So many people are stuck with a partner with whom they can't speak. One possibility for such people is to back off. Detach yourself emotionally and you still can have passionate sex life. I'm not sure if that's also true if the partner lost attractiveness already. I guess for sex with an ugly person emotional attachment is necessary. Also for people with capacity for attachment I doubt that's fulfilling, even if the partner looks good.
I guess St. Paul had this kind of sex focused/ back off relationship in mind when he wrote in 1 Corinthians 7:
"Now for the matters you wrote about: “It is good for a man not to have sexual relations with a woman.” 2 But since sexual immorality is occurring, each man should have sexual relations with his own wife, and each woman with her own husband. 3 The husband should fulfill his marital duty to his wife, and likewise the wife to her husband. 4 The wife does not have authority over her own body but yields it to her husband. In the same way, the husband does not have authority over his own body but yields it to his wife. 5 Do not deprive each other except perhaps by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control. 6 I say this as a concession, not as a command. 7 I wish that all of you were as I am. But each of you has your own gift from God; one has this gift, another has that."
Then there are people born to be disinterested:
"Schizoid personality disorder (/ˈskɪtsɔɪd, ˈskɪdzɔɪd/, often abbreviated as SPD or SzPD) is a personality disorder characterized by a lack of interest in social relationships, a tendency toward a solitary or sheltered lifestyle, secretiveness, emotional coldness, detachment and apathy. Affected individuals may be unable to form intimate attachments to others and simultaneously possess a rich and elaborate but exclusively internal fantasy world. Other associated features include stilted speech, a lack of deriving enjoyment from most activities, feeling as though one is an "observer" rather than a participant in life, an inability to tolerate emotional expectations of others, apparent indifference when praised or criticized, a degree of asexuality and idiosyncratic moral or political beliefs."https://en.wikipedia.org/wiki/Schizoid_ ... y_disorder
In individualistic countries hard cases won't marry. However for one person meeting diagnostic criteria there are several persons having such character. Some of the mild cases will marry.
Then there is hyposexuality:
If the partner is a pain in the ass it's likely even a man will loose interest in sex. Change the partner and he's alright again.
Somebody who cheats on his partner may also loose interest in sex.
We humans have several contradicting characteristics.
1) We want to be superior to others (e.g. better car) and at the same time we need cooperation to be happy. We are better monkeys. Can everybody be superior?
2) We want to be polygam. But it doesn't do most of us any good IF we are in a functional relationship.
Then there can be hormonal reasons for hyposexuality.https://en.wikipedia.org/wiki/Hypoactiv ... e_disorder
So "one has to discuss" and "one has to compromise" are brainwashs. Mental healthy people capable of attachment who have a partner with sound mental health don't need such advise. For those who have a partner with mental health problems the advise is harmful.
I'm not sure if there are people affected with personality disorders for whom that advise can be of help. I tend to believe "No".
If married sex life is bad I assume it mostly means the relationship is not functional. Bad sex is a symptom, not the problem.